diff --git a/.env.example b/.env.example
new file mode 100644
index 0000000..6fabb56
--- /dev/null
+++ b/.env.example
@@ -0,0 +1,12 @@
+# Database driver: mysql or sqlite (tests use sqlite)
+QDB_DRIVER=mysql
+
+# MySQL connection (required when QDB_DRIVER=mysql)
+QDB_MYSQL_HOST=127.0.0.1
+QDB_MYSQL_PORT=3306
+QDB_MYSQL_DATABASE=nat-as-db
+QDB_MYSQL_USER=nat_as_db
+QDB_MYSQL_PASSWORD=change-me
+
+# AES key for legacy SQLite file encryption and app payload crypto (32 bytes, base64)
+QDB_MASTER_KEY=
diff --git a/.github/workflows/phpunit.yml b/.github/workflows/phpunit.yml
index bf175d4..dc9073c 100644
--- a/.github/workflows/phpunit.yml
+++ b/.github/workflows/phpunit.yml
@@ -9,6 +9,22 @@ jobs:
   test:
     runs-on: ubuntu-latest
 
+    services:
+      mysql:
+        image: mysql:8.4
+        env:
+          MYSQL_DATABASE: nat_as_test
+          MYSQL_USER: nat_as_test
+          MYSQL_PASSWORD: test-password
+          MYSQL_ROOT_PASSWORD: root-password
+        ports:
+          - 3306:3306
+        options: >-
+          --health-cmd="mysqladmin ping -h 127.0.0.1 -uroot -proot-password"
+          --health-interval=5s
+          --health-timeout=5s
+          --health-retries=20
+
     steps:
       - uses: actions/checkout@v4
 
@@ -16,7 +32,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: '8.2'
-          extensions: json, openssl, pdo, pdo_sqlite, zip
+          extensions: json, openssl, pdo, pdo_mysql, pdo_sqlite, zip
           coverage: pcov
           tools: composer:v2
 
@@ -25,3 +41,12 @@ jobs:
 
       - name: Run tests with coverage floor
         run: php tests/check-coverage.php 40
+
+      - name: Run MySQL schema smoke test
+        env:
+          QDB_MYSQL_HOST: 127.0.0.1
+          QDB_MYSQL_PORT: 3306
+          QDB_MYSQL_DATABASE: nat_as_test
+          QDB_MYSQL_USER: nat_as_test
+          QDB_MYSQL_PASSWORD: test-password
+        run: php tests/mysql_smoke.php
diff --git a/cli/diagnose_db.php b/cli/diagnose_db.php
index 347d08a..135e5b2 100644
--- a/cli/diagnose_db.php
+++ b/cli/diagnose_db.php
@@ -36,11 +36,37 @@ echo ".env: " . ($envPath && is_readable($envPath) ? "readable" : "MISSING or no
 $keyEnv = getenv('QDB_MASTER_KEY');
 if ($keyEnv === false || $keyEnv === '') {
     echo "QDB_MASTER_KEY: NOT SET (set in .env or Apache/PHP-FPM env)\n";
-    exit(1);
+    if (!qdb_is_mysql()) {
+        exit(1);
+    }
+} else {
+    echo "QDB_MASTER_KEY: set (" . strlen($keyEnv) . " chars)\n";
+    $b64 = base64_decode($keyEnv, true);
+    echo "  parses as base64: " . ($b64 !== false && strlen($b64) > 0 ? 'yes (' . strlen($b64) . ' bytes)' : 'no (uses first 32 ASCII chars)') . "\n";
 }
-echo "QDB_MASTER_KEY: set (" . strlen($keyEnv) . " chars)\n";
-$b64 = base64_decode($keyEnv, true);
-echo "  parses as base64: " . ($b64 !== false && strlen($b64) > 0 ? 'yes (' . strlen($b64) . ' bytes)' : 'no (uses first 32 ASCII chars)') . "\n";
+
+if (qdb_is_mysql()) {
+    echo "QDB_DRIVER: mysql\n";
+    echo "  host: " . (qdb_env_get('QDB_MYSQL_HOST') ?: '127.0.0.1') . "\n";
+    echo "  database: " . (qdb_env_get('QDB_MYSQL_DATABASE') ?: 'nat-as-db') . "\n";
+    try {
+        [$pdo] = qdb_mysql_open();
+        qdb_mysql_ensure_ready($pdo);
+        $version = qdb_schema_version($pdo);
+        $users = (int)$pdo->query('SELECT COUNT(*) FROM users')->fetchColumn();
+        $hasSession = qdb_table_exists($pdo, 'session');
+        echo "\nOK: MySQL connection + migrations succeeded.\n";
+        echo "  schema_version: $version (expected " . QDB_VERSION . ")\n";
+        echo "  users: $users\n";
+        echo "  session table: " . ($hasSession ? 'yes' : 'MISSING') . "\n";
+        exit(0);
+    } catch (Throwable $e) {
+        echo "\nMySQL open failed: " . $e->getMessage() . "\n";
+        exit(1);
+    }
+}
+
+echo "QDB_DRIVER: sqlite\n";
 
 $dbPath = $argv[1] ?? QDB_PATH;
 if ($dbPath !== QDB_PATH && !is_file($dbPath)) {
diff --git a/cli/migrate_sqlite_to_mysql.php b/cli/migrate_sqlite_to_mysql.php
new file mode 100644
index 0000000..249a43a
--- /dev/null
+++ b/cli/migrate_sqlite_to_mysql.php
@@ -0,0 +1,82 @@
+#!/usr/bin/env php
+<?php
+
+declare(strict_types=1);
+
+/**
+ * One-time migration: encrypted SQLite file -> MySQL database.
+ *
+ * Usage:
+ *   php cli/migrate_sqlite_to_mysql.php
+ *
+ * Requires QDB_MASTER_KEY (source SQLite) and QDB_MYSQL_* variables (target).
+ */
+
+require_once __DIR__ . '/../vendor/autoload.php';
+require_once __DIR__ . '/../lib/response.php';
+require_once __DIR__ . '/../common.php';
+qdb_env_override('QDB_DRIVER', 'sqlite');
+require_once __DIR__ . '/../db_init.php';
+
+if (!is_file(QDB_PATH)) {
+    fwrite(STDERR, "SQLite database file not found at " . QDB_PATH . PHP_EOL);
+    exit(1);
+}
+
+[$sqlitePdo, $tmpDb, $lockFp] = qdb_open(false);
+
+$tables = $sqlitePdo->query(
+    "SELECT name FROM sqlite_master WHERE type = 'table' AND name NOT LIKE 'sqlite_%' ORDER BY name"
+)->fetchAll(PDO::FETCH_COLUMN);
+
+$tableData = [];
+foreach ($tables as $table) {
+    $tableData[$table] = $sqlitePdo->query("SELECT * FROM `{$table}`")->fetchAll(PDO::FETCH_ASSOC);
+}
+qdb_discard($tmpDb, $lockFp);
+
+qdb_reset_driver_cache();
+qdb_env_override('QDB_DRIVER', 'mysql');
+
+[$mysqlPdo] = qdb_mysql_open();
+qdb_mysql_ensure_ready($mysqlPdo);
+
+qdb_set_foreign_keys($mysqlPdo, false);
+$mysqlPdo->beginTransaction();
+
+try {
+    foreach ($tableData as $table => $rows) {
+        if (!qdb_table_exists($mysqlPdo, $table)) {
+            throw new RuntimeException("MySQL schema is missing source table: {$table}");
+        }
+        $mysqlPdo->exec("DELETE FROM `{$table}`");
+        if (!$rows) {
+            echo "{$table}: 0 rows\n";
+            continue;
+        }
+        $columns = array_keys($rows[0]);
+        $colList = implode(', ', array_map(fn($c) => "`{$c}`", $columns));
+        $placeholders = implode(', ', array_map(fn($c) => ':' . $c, $columns));
+        $insert = $mysqlPdo->prepare("INSERT INTO `{$table}` ({$colList}) VALUES ({$placeholders})");
+        foreach ($rows as $row) {
+            $params = [];
+            foreach ($columns as $column) {
+                $params[':' . $column] = $row[$column];
+            }
+            $insert->execute($params);
+        }
+        echo "{$table}: " . count($rows) . " rows\n";
+    }
+    $mysqlPdo->commit();
+} catch (Throwable $e) {
+    if ($mysqlPdo->inTransaction()) {
+        $mysqlPdo->rollBack();
+    }
+    throw $e;
+} finally {
+    qdb_set_foreign_keys($mysqlPdo, true);
+}
+
+qdb_set_schema_version($mysqlPdo, QDB_VERSION);
+
+echo "Migration complete. Set QDB_DRIVER=mysql in .env and restart PHP.\n";
diff --git a/common.php b/common.php
index 00c2445..671eca2 100644
--- a/common.php
+++ b/common.php
@@ -76,6 +76,9 @@ function qdb_parse_env_file(string $path): array {
  * Read an environment variable (.env file first, then getenv / $_ENV / $_SERVER).
  */
 function qdb_env_get(string $name): ?string {
+    if (isset($GLOBALS['qdb_env_overrides'][$name])) {
+        return (string)$GLOBALS['qdb_env_overrides'][$name];
+    }
     $fileVars = qdb_parse_env_file(qdb_env_file_path());
     if (isset($fileVars[$name]) && $fileVars[$name] !== '') {
         return $fileVars[$name];
@@ -110,6 +113,15 @@ function qdb_env_set(string $name, string $value): void {
     }
 }
 
+/** Override a configuration value for the current process, ahead of .env. */
+function qdb_env_override(string $name, string $value): void {
+    $GLOBALS['qdb_env_overrides'][$name] = $value;
+    qdb_env_set($name, $value);
+    if ($name === 'QDB_DRIVER' && function_exists('qdb_reset_driver_cache')) {
+        qdb_reset_driver_cache();
+    }
+}
+
 /** Load KEY=value pairs from .env into $GLOBALS['qdb_dotenv'] and $_ENV. */
 function qdb_load_dotenv(string $path): void {
     foreach (qdb_parse_env_file($path) as $name => $value) {
@@ -382,19 +394,18 @@ define('QDB_SOURCE_LANGUAGE', 'de');
 
 function qdb_ensure_source_language(PDO $pdo): void {
     if (qdb_column_exists($pdo, 'language', 'enabled')) {
-        $pdo->prepare("INSERT INTO language (languageCode, name, enabled) VALUES (:lc, :n, 1)
-                       ON CONFLICT(languageCode) DO UPDATE SET enabled = 1")
+        $pdo->prepare('INSERT INTO language (languageCode, name, enabled) VALUES (:lc, :n, 1) '
+            . qdb_upsert_update(['languageCode'], ['enabled' => '1', 'name' => 'excluded.name']))
             ->execute([':lc' => QDB_SOURCE_LANGUAGE, ':n' => 'German']);
         return;
     }
-    $pdo->prepare("INSERT INTO language (languageCode, name) VALUES (:lc, :n)
-                   ON CONFLICT(languageCode) DO NOTHING")
+    $pdo->prepare('INSERT INTO language (languageCode, name) VALUES (:lc, :n) '
+        . qdb_upsert_do_nothing(['languageCode']))
         ->execute([':lc' => QDB_SOURCE_LANGUAGE, ':n' => 'German']);
 }
 
 /** @return list<array{languageCode:string,name:string,enabled:int}> */
 function qdb_load_languages(PDO $pdo): array {
-    qdb_ensure_source_language($pdo);
     $hasEnabled = qdb_column_exists($pdo, 'language', 'enabled');
     if ($hasEnabled) {
         $rows = $pdo->query(
@@ -439,9 +450,9 @@ function qdb_set_language_enabled(PDO $pdo, string $languageCode, bool $enabled)
     if (!qdb_column_exists($pdo, 'language', 'enabled')) {
         throw new RuntimeException('Language enablement is not available on this database');
     }
-    $pdo->prepare('UPDATE language SET enabled = :en WHERE languageCode = :lc')
-        ->execute([':en' => $enabled ? 1 : 0, ':lc' => $languageCode]);
-    if ($pdo->query('SELECT changes()')->fetchColumn() == 0) {
+    $stmt = $pdo->prepare('UPDATE language SET enabled = :en WHERE languageCode = :lc');
+    $stmt->execute([':en' => $enabled ? 1 : 0, ':lc' => $languageCode]);
+    if (qdb_stmt_affected_rows($pdo, $stmt) === 0) {
         throw new InvalidArgumentException('Language not found');
     }
 }
@@ -496,7 +507,7 @@ function qdb_set_questionnaire_language_disabled(
     if ($disabled) {
         $pdo->prepare(
             'INSERT INTO questionnaire_language_disable (questionnaireID, languageCode)
-             VALUES (:qid, :lc) ON CONFLICT(questionnaireID, languageCode) DO NOTHING'
+             VALUES (:qid, :lc) ' . qdb_upsert_do_nothing(['questionnaireID', 'languageCode'])
         )->execute([':qid' => $questionnaireID, ':lc' => $languageCode]);
         return;
     }
@@ -1161,24 +1172,24 @@ function qdb_app_ui_string_entries_for_website(PDO $pdo): array {
 /** Upsert one translation row; sync German text into defaultText for questions/options. */
 function qdb_put_translation(PDO $pdo, string $type, string $id, string $lang, string $text): void {
     if ($type === 'question') {
-        $pdo->prepare("INSERT INTO question_translation (questionID, languageCode, text)
-                       VALUES (:id, :lang, :t)
-                       ON CONFLICT(questionID, languageCode) DO UPDATE SET text = excluded.text")
+        $pdo->prepare('INSERT INTO question_translation (questionID, languageCode, text)
+                       VALUES (:id, :lang, :t) '
+            . qdb_upsert_update(['questionID', 'languageCode'], ['text' => 'excluded.text']))
             ->execute([':id' => $id, ':lang' => $lang, ':t' => $text]);
         if ($lang === QDB_SOURCE_LANGUAGE) {
             $pdo->prepare("UPDATE question SET defaultText = :t WHERE questionID = :id")
                 ->execute([':t' => $text, ':id' => $id]);
         }
     } elseif ($type === 'answer_option') {
-        $pdo->prepare("INSERT INTO answer_option_translation (answerOptionID, languageCode, text)
-                       VALUES (:id, :lang, :t)
-                       ON CONFLICT(answerOptionID, languageCode) DO UPDATE SET text = excluded.text")
+        $pdo->prepare('INSERT INTO answer_option_translation (answerOptionID, languageCode, text)
+                       VALUES (:id, :lang, :t) '
+            . qdb_upsert_update(['answerOptionID', 'languageCode'], ['text' => 'excluded.text']))
             ->execute([':id' => $id, ':lang' => $lang, ':t' => $text]);
         // defaultText holds optionKey; German label lives only in answer_option_translation.
     } elseif ($type === 'string' || $type === 'app_string') {
-        $pdo->prepare("INSERT INTO string_translation (stringKey, languageCode, text)
-                       VALUES (:id, :lang, :t)
-                       ON CONFLICT(stringKey, languageCode) DO UPDATE SET text = excluded.text")
+        $pdo->prepare('INSERT INTO string_translation (stringKey, languageCode, text)
+                       VALUES (:id, :lang, :t) '
+            . qdb_upsert_update(['stringKey', 'languageCode'], ['text' => 'excluded.text']))
             ->execute([':id' => $id, ':lang' => $lang, ':t' => $text]);
     }
 }
@@ -2500,8 +2511,8 @@ function qdb_import_translations_bundle(PDO $pdo, array $bundle): array {
         if ($lc === '' || $lc === QDB_SOURCE_LANGUAGE) {
             continue;
         }
-        $pdo->prepare('INSERT INTO language (languageCode, name) VALUES (:lc, :n)
-                       ON CONFLICT(languageCode) DO UPDATE SET name = excluded.name')
+        $pdo->prepare('INSERT INTO language (languageCode, name) VALUES (:lc, :n) '
+            . qdb_upsert_update(['languageCode'], ['name' => 'excluded.name']))
             ->execute([':lc' => $lc, ':n' => $name]);
     }
 
diff --git a/composer.json b/composer.json
index ad7f717..8952cba 100644
--- a/composer.json
+++ b/composer.json
@@ -1,12 +1,13 @@
 {
     "name": "nat-as/questionnaire-server",
-    "description": "Encrypted SQLite questionnaire API",
+    "description": "Questionnaire API (MySQL or legacy encrypted SQLite)",
     "type": "project",
     "require": {
         "php": ">=8.2",
         "ext-json": "*",
         "ext-openssl": "*",
         "ext-pdo": "*",
+        "ext-pdo_mysql": "*",
         "ext-pdo_sqlite": "*"
     },
     "require-dev": {
diff --git a/db_init.php b/db_init.php
index 18311a8..07a0031 100644
--- a/db_init.php
+++ b/db_init.php
@@ -1,7 +1,7 @@
 <?php
-// /var/www/html/db_init.php
-// Shared helpers for opening, creating, and saving the encrypted SQLite database.
+// Shared helpers for opening, creating, and saving the questionnaire database.
 require_once __DIR__ . '/common.php';
+require_once __DIR__ . '/lib/sql_dialect.php';
 
 if (defined('QDB_TEST_UPLOADS')) {
     define('QDB_UPLOADS_DIR', QDB_TEST_UPLOADS);
@@ -15,26 +15,8 @@ if (defined('QDB_TEST_UPLOADS')) {
 define('QDB_SCHEMA', __DIR__ . '/schema.sql');
 define('QDB_VERSION', 14);
 
-function qdb_table_exists(PDO $pdo, string $table): bool {
-    $stmt = $pdo->prepare(
-        "SELECT 1 FROM sqlite_master WHERE type = 'table' AND name = :t LIMIT 1"
-    );
-    $stmt->execute([':t' => $table]);
-    return (bool)$stmt->fetchColumn();
-}
-
-function qdb_column_exists(PDO $pdo, string $table, string $column): bool {
-    $stmt = $pdo->query('PRAGMA table_info(' . $table . ')');
-    foreach ($stmt->fetchAll(PDO::FETCH_ASSOC) as $col) {
-        if (($col['name'] ?? '') === $column) {
-            return true;
-        }
-    }
-    return false;
-}
-
 /**
- * Apply incremental schema fixes. Returns true if the temp DB was modified.
+ * Apply incremental schema fixes. Returns true if the DB was modified.
  */
 function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
     $changed = false;
@@ -138,8 +120,8 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
                 UNIQUE(clientCode, questionnaireID, version)
             )
         ");
-        $pdo->exec('CREATE INDEX IF NOT EXISTS idx_submission_client_qn ON questionnaire_submission(clientCode, questionnaireID)');
-        $pdo->exec('CREATE INDEX IF NOT EXISTS idx_submission_client_time ON questionnaire_submission(clientCode, submittedAt)');
+        qdb_create_index_if_not_exists($pdo, 'idx_submission_client_qn', 'questionnaire_submission', 'clientCode, questionnaireID');
+        qdb_create_index_if_not_exists($pdo, 'idx_submission_client_time', 'questionnaire_submission', 'clientCode, submittedAt');
         $changed = true;
     }
 
@@ -211,7 +193,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
                 UNIQUE(questionID, scopeKey, levelKey)
             )
         ");
-        $pdo->exec('CREATE INDEX IF NOT EXISTS idx_score_rule_question ON question_score_rule(questionID)');
+        qdb_create_index_if_not_exists($pdo, 'idx_score_rule_question', 'question_score_rule', 'questionID');
         $changed = true;
     }
 
@@ -267,7 +249,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
                 FOREIGN KEY(profileID)  REFERENCES scoring_profile(profileID) ON DELETE CASCADE
             )
         ");
-        $pdo->exec('CREATE INDEX IF NOT EXISTS idx_client_profile_result_profile ON client_scoring_profile_result(profileID)');
+        qdb_create_index_if_not_exists($pdo, 'idx_client_profile_result_profile', 'client_scoring_profile_result', 'profileID');
         $changed = true;
     }
 
@@ -293,7 +275,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
         }
     }
 
-    $currentVersion = (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+    $currentVersion = qdb_schema_version($pdo);
     if ($currentVersion < 10
         && qdb_table_exists($pdo, 'completed_questionnaire')
         && qdb_table_exists($pdo, 'client_scoring_profile_result')) {
@@ -303,7 +285,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
         }
     }
 
-    $currentVersion = (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+    $currentVersion = qdb_schema_version($pdo);
     if ($currentVersion < 7 && qdb_table_exists($pdo, 'question_score_rule')) {
         require_once __DIR__ . '/lib/scoring.php';
         if (qdb_backfill_glass_score_rules($pdo) > 0) {
@@ -317,7 +299,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
         }
     }
 
-    $currentVersion = (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+    $currentVersion = qdb_schema_version($pdo);
     if ($currentVersion < 8 && qdb_table_exists($pdo, 'scoring_profile')) {
         if (!qdb_column_exists($pdo, 'scoring_profile', 'greenMin')) {
             $pdo->exec('ALTER TABLE scoring_profile ADD COLUMN greenMin INTEGER NOT NULL DEFAULT 0');
@@ -340,7 +322,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
         $changed = true;
     }
 
-    $currentVersion = (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+    $currentVersion = qdb_schema_version($pdo);
     if ($currentVersion < 13 && qdb_table_exists($pdo, 'scoring_profile')) {
         if (!qdb_column_exists($pdo, 'scoring_profile', 'processCompleteBands')) {
             $pdo->exec("ALTER TABLE scoring_profile ADD COLUMN processCompleteBands TEXT NOT NULL DEFAULT '[]'");
@@ -348,7 +330,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
         }
     }
 
-    $currentVersion = (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+    $currentVersion = qdb_schema_version($pdo);
     if ($currentVersion < 14) {
         if (qdb_table_exists($pdo, 'language') && !qdb_column_exists($pdo, 'language', 'enabled')) {
             $pdo->exec('ALTER TABLE language ADD COLUMN enabled INTEGER NOT NULL DEFAULT 1');
@@ -368,7 +350,7 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
         }
     }
 
-    $currentVersion = (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+    $currentVersion = qdb_schema_version($pdo);
     if ($currentVersion < 6 && qdb_table_exists($pdo, 'questionnaire_submission')) {
         $pdo->exec(
             'UPDATE questionnaire_submission SET submittedAt = completedAt
@@ -377,27 +359,55 @@ function qdb_apply_migrations(PDO $pdo, string $schemaSql): bool {
         $changed = true;
     }
 
-    $currentVersion = (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+    $currentVersion = qdb_schema_version($pdo);
     if ($currentVersion < QDB_VERSION) {
-        $pdo->exec('PRAGMA foreign_keys = OFF;');
-        $pdo->exec($schemaSql);
-        if (!qdb_column_exists($pdo, 'questionnaire', 'categoryKey')) {
-            $pdo->exec("ALTER TABLE questionnaire ADD COLUMN categoryKey TEXT NOT NULL DEFAULT ''");
+        qdb_set_foreign_keys($pdo, false);
+        try {
+            qdb_exec_schema_sql($pdo, $schemaSql);
+            if (!qdb_column_exists($pdo, 'questionnaire', 'categoryKey')) {
+                $pdo->exec("ALTER TABLE questionnaire ADD COLUMN categoryKey TEXT NOT NULL DEFAULT ''");
+            }
+            qdb_set_schema_version($pdo, QDB_VERSION);
+        } finally {
+            qdb_set_foreign_keys($pdo, true);
         }
-        $pdo->exec('PRAGMA user_version = ' . QDB_VERSION . ';');
-        $pdo->exec('PRAGMA foreign_keys = ON;');
         $changed = true;
     }
 
     return $changed;
 }
 
-/**
- * Exclusive lock with short retries (non-blocking attempts). Avoids hung PHP workers
- * when two writes overlap (app upload + website edit).
- */
-function qdb_flock_exclusive_with_retry($lockFp, int $maxAttempts = 12, int $sleepMicros = 75000): void
-{
+function qdb_mysql_ensure_ready(PDO $pdo): void {
+    static $ready = false;
+    if ($ready) {
+        return;
+    }
+
+    $lockName = 'nat_as_schema_migration';
+    $lockStmt = $pdo->prepare('SELECT GET_LOCK(:name, 30)');
+    $lockStmt->execute([':name' => $lockName]);
+    if ((int)$lockStmt->fetchColumn() !== 1) {
+        throw new RuntimeException('Could not acquire MySQL schema migration lock');
+    }
+
+    try {
+        $schemaPath = qdb_schema_file();
+        $schemaSql = file_get_contents($schemaPath);
+        if ($schemaSql === false) {
+            throw new Exception('Could not read MySQL schema');
+        }
+        if (!qdb_table_exists($pdo, 'users') || qdb_schema_version($pdo) < QDB_VERSION) {
+            qdb_exec_schema_file($pdo, $schemaPath);
+        }
+        qdb_apply_migrations($pdo, $schemaSql);
+        $ready = true;
+    } finally {
+        $releaseStmt = $pdo->prepare('SELECT RELEASE_LOCK(:name)');
+        $releaseStmt->execute([':name' => $lockName]);
+    }
+}
+
+function qdb_flock_exclusive_with_retry($lockFp, int $maxAttempts = 12, int $sleepMicros = 75000): void {
     for ($attempt = 0; $attempt < $maxAttempts; $attempt++) {
         if (flock($lockFp, LOCK_EX | LOCK_NB)) {
             return;
@@ -409,8 +419,7 @@ function qdb_flock_exclusive_with_retry($lockFp, int $maxAttempts = 12, int $sle
     throw new Exception('Could not acquire lock');
 }
 
-function qdb_open_writable_lock()
-{
+function qdb_open_writable_lock() {
     $lockFp = fopen(QDB_LOCK, 'c');
     if ($lockFp === false) {
         throw new Exception('Could not open lock file');
@@ -424,14 +433,15 @@ function qdb_acquire_lock() {
 }
 
 /**
- * Decrypt the master DB file into a temp SQLite file, or create a fresh one
- * from schema.sql if no DB exists yet.
- *
  * Returns [PDO $pdo, string $tmpPath, resource|null $lockFp].
- * If $writable is true, an exclusive lock is held -- caller MUST call
- * qdb_save() or qdb_discard() afterwards.
  */
 function qdb_open(bool $writable = false): array {
+    if (qdb_is_mysql()) {
+        [$pdo] = qdb_mysql_open();
+        qdb_mysql_ensure_ready($pdo);
+        return [$pdo, '', null];
+    }
+
     if (!$writable) {
         require_once __DIR__ . '/lib/read_db_cache.php';
         return qdb_read_db_open();
@@ -441,29 +451,28 @@ function qdb_open(bool $writable = false): array {
 
     if (!is_dir(dirname($dbPath))) {
         if (!mkdir(dirname($dbPath), 0755, true) && !is_dir(dirname($dbPath))) {
-            throw new Exception("Could not create uploads directory");
+            throw new Exception('Could not create uploads directory');
         }
     }
 
-    $lockFp = null;
-    if ($writable) {
-        $lockFp = qdb_open_writable_lock();
-    }
+    $lockFp = qdb_open_writable_lock();
 
     $tmpDb = tempnam(sys_get_temp_dir(), 'qdb_');
     $masterKey = get_master_key_bytes();
 
     $sql = file_get_contents(QDB_SCHEMA);
     if ($sql === false) {
-        if ($lockFp) { flock($lockFp, LOCK_UN); fclose($lockFp); }
-        throw new Exception("Could not read schema.sql");
+        flock($lockFp, LOCK_UN);
+        fclose($lockFp);
+        throw new Exception('Could not read schema.sql');
     }
 
     if (file_exists($dbPath) && is_file($dbPath)) {
         $storedEnc = @file_get_contents($dbPath);
         if ($storedEnc === false) {
-            if ($lockFp) { flock($lockFp, LOCK_UN); fclose($lockFp); }
-            throw new Exception("Could not read stored DB");
+            flock($lockFp, LOCK_UN);
+            fclose($lockFp);
+            throw new Exception('Could not read stored DB');
         }
         try {
             $decrypted = aes256_cbc_decrypt_bytes($storedEnc, $masterKey);
@@ -481,45 +490,32 @@ function qdb_open(bool $writable = false): array {
             throw $e;
         }
         if (file_put_contents($tmpDb, $decrypted) === false) {
-            if ($lockFp) { flock($lockFp, LOCK_UN); fclose($lockFp); }
-            throw new Exception("Could not write temp DB");
+            flock($lockFp, LOCK_UN);
+            fclose($lockFp);
+            throw new Exception('Could not write temp DB');
         }
     } else {
         $pdo = new PDO("sqlite:$tmpDb");
         $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
         $pdo->exec($sql);
-        $pdo->exec("PRAGMA user_version = " . QDB_VERSION . ";");
+        qdb_set_schema_version($pdo, QDB_VERSION);
         $pdo = null;
     }
 
     $pdo = new PDO("sqlite:$tmpDb");
     $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
-    $pdo->exec("PRAGMA foreign_keys = ON;");
+    qdb_set_foreign_keys($pdo, true);
 
-    $schemaChanged = qdb_apply_migrations($pdo, $sql);
-
-    // Read-only opens discard the temp file; persist one-off migrations to the master DB.
-    if ($schemaChanged && !$writable) {
-        $migrateLock = qdb_acquire_lock();
-        try {
-            qdb_save($tmpDb, $migrateLock);
-        } catch (Throwable $e) {
-            @unlink($tmpDb);
-            flock($migrateLock, LOCK_UN);
-            fclose($migrateLock);
-            throw $e;
-        }
-        return qdb_open(false);
-    }
+    qdb_apply_migrations($pdo, $sql);
 
     return [$pdo, $tmpDb, $lockFp];
 }
 
-/**
- * Encrypt the temp DB and atomically write it back to the master path.
- * Releases the lock.
- */
 function qdb_save(string $tmpDb, $lockFp): void {
+    if (qdb_is_mysql()) {
+        return;
+    }
+
     $masterKey = get_master_key_bytes();
     try {
         $ck = new PDO('sqlite:' . $tmpDb);
@@ -528,17 +524,19 @@ function qdb_save(string $tmpDb, $lockFp): void {
         // best-effort before reading plaintext bytes
     }
     $plainDb = file_get_contents($tmpDb);
-    if ($plainDb === false) throw new Exception("Could not read temp DB for save");
+    if ($plainDb === false) {
+        throw new Exception('Could not read temp DB for save');
+    }
     $enc = aes256_cbc_encrypt_bytes($plainDb, $masterKey);
 
     $dbPath = QDB_PATH;
     $tmpEncrypted = tempnam(dirname($dbPath), 'enc_qdb_');
     if (file_put_contents($tmpEncrypted, $enc) === false) {
-        throw new Exception("Could not write encrypted DB");
+        throw new Exception('Could not write encrypted DB');
     }
     if (!@rename($tmpEncrypted, $dbPath)) {
         if (!@copy($tmpEncrypted, $dbPath) || !@unlink($tmpEncrypted)) {
-            throw new Exception("Could not save encrypted DB (rename/copy failed)");
+            throw new Exception('Could not save encrypted DB (rename/copy failed)');
         }
     }
     @chmod($dbPath, 0644);
@@ -553,10 +551,10 @@ function qdb_save(string $tmpDb, $lockFp): void {
     qdb_read_cache_invalidate();
 }
 
-/**
- * Discard changes -- clean up temp file and release lock without saving.
- */
 function qdb_discard(string $tmpDb, $lockFp): void {
+    if (qdb_is_mysql()) {
+        return;
+    }
     if (defined('QDB_READONLY_CACHE_HANDLE') && $tmpDb === QDB_READONLY_CACHE_HANDLE) {
         return;
     }
@@ -567,14 +565,7 @@ function qdb_discard(string $tmpDb, $lockFp): void {
     }
 }
 
-/**
- * Open a read-only DB connection; on failure logs and returns a JSON error response.
- *
- * @return array{0: PDO, 1: string, 2: resource|null}|null
- */
-/**
- * @return array{0: PDO, 1: string, 2: resource|null}
- */
+/** @return array{0: PDO, 1: string, 2: resource|null} */
 function qdb_open_read_or_fail(): array {
     try {
         return qdb_open(false);
@@ -584,9 +575,7 @@ function qdb_open_read_or_fail(): array {
     }
 }
 
-/**
- * @return array{0: PDO, 1: string, 2: resource|null}
- */
+/** @return array{0: PDO, 1: string, 2: resource|null} */
 function qdb_open_write_or_fail(): array {
     try {
         return qdb_open(true);
diff --git a/handlers/answer_options.php b/handlers/answer_options.php
index 0065a9c..e0e36b5 100644
--- a/handlers/answer_options.php
+++ b/handlers/answer_options.php
@@ -181,10 +181,10 @@ case 'DELETE':
             json_success(['deleted' => false, 'retired' => true, 'structureRevision' => $newRev]);
             break;
         }
-        $pdo->exec('PRAGMA foreign_keys = OFF;');
+        qdb_set_foreign_keys($pdo, false);
         $pdo->prepare('DELETE FROM answer_option_translation WHERE answerOptionID = :id')->execute([':id' => $id]);
         $pdo->prepare('DELETE FROM answer_option WHERE answerOptionID = :id')->execute([':id' => $id]);
-        $pdo->exec('PRAGMA foreign_keys = ON;');
+        qdb_set_foreign_keys($pdo, true);
         $pdo->commit();
         qdb_save($tmpDb, $lockFp);
         json_success(['deleted' => true, 'retired' => false, 'structureRevision' => $newRev]);
diff --git a/handlers/app_questionnaires.php b/handlers/app_questionnaires.php
index c978a0d..d9a3a6e 100644
--- a/handlers/app_questionnaires.php
+++ b/handlers/app_questionnaires.php
@@ -180,15 +180,19 @@ if ($method === 'POST') {
 
         $glassByParent = [];
         $submittedQuestionIds = [];
-        $answerInsert = $pdo->prepare("
+        $answerInsert = $pdo->prepare('
             INSERT INTO client_answer (clientCode, questionID, answerOptionID, freeTextValue, numericValue, answeredAt)
-            VALUES (:cc, :qid, :aoid, :ftv, :nv, :at)
-            ON CONFLICT(clientCode, questionID) DO UPDATE SET
-                answerOptionID = excluded.answerOptionID,
-                freeTextValue  = excluded.freeTextValue,
-                numericValue   = excluded.numericValue,
-                answeredAt     = excluded.answeredAt
-        ");
+            VALUES (:cc, :qid, :aoid, :ftv, :nv, :at) '
+            . qdb_upsert_update(
+                ['clientCode', 'questionID'],
+                [
+                    'answerOptionID' => 'excluded.answerOptionID',
+                    'freeTextValue' => 'excluded.freeTextValue',
+                    'numericValue' => 'excluded.numericValue',
+                    'answeredAt' => 'excluded.answeredAt',
+                ]
+            )
+        );
         $answerDelete = $pdo->prepare(
             'DELETE FROM client_answer WHERE clientCode = :cc AND questionID = :qid'
         );
@@ -298,16 +302,20 @@ if ($method === 'POST') {
             ? qdb_compute_questionnaire_score_from_manifest($pdo, $clientCode, $submitManifest)
             : qdb_compute_questionnaire_score($pdo, $clientCode, $qnID);
 
-        $pdo->prepare("
+        $pdo->prepare('
             INSERT INTO completed_questionnaire (clientCode, questionnaireID, assignedByCoach, status, startedAt, completedAt, sumPoints)
-            VALUES (:cc, :qn, :abc, 'completed', :sa, :ca, :sp)
-            ON CONFLICT(clientCode, questionnaireID) DO UPDATE SET
-                assignedByCoach = excluded.assignedByCoach,
-                status          = 'completed',
-                startedAt       = COALESCE(excluded.startedAt, startedAt),
-                completedAt     = excluded.completedAt,
-                sumPoints       = excluded.sumPoints
-        ")->execute([
+            VALUES (:cc, :qn, :abc, \'completed\', :sa, :ca, :sp) '
+            . qdb_upsert_update(
+                ['clientCode', 'questionnaireID'],
+                [
+                    'assignedByCoach' => 'excluded.assignedByCoach',
+                    'status' => "'completed'",
+                    'startedAt' => 'COALESCE(excluded.startedAt, startedAt)',
+                    'completedAt' => 'excluded.completedAt',
+                    'sumPoints' => 'excluded.sumPoints',
+                ]
+            )
+        )->execute([
             ':cc'  => $clientCode,
             ':qn'  => $qnID,
             ':abc' => $assignedByCoach !== '' ? $assignedByCoach : null,
diff --git a/handlers/backup.php b/handlers/backup.php
index a8a8e3d..efb8cad 100644
--- a/handlers/backup.php
+++ b/handlers/backup.php
@@ -7,21 +7,70 @@ if ($method !== 'POST') {
     json_error('METHOD_NOT_ALLOWED', 'Method not allowed', 405);
 }
 
-$dbPath    = QDB_PATH;
 $backupDir = QDB_UPLOADS_DIR . '/backups';
-
-if (!file_exists($dbPath)) {
-    json_error('NOT_FOUND', 'No database to back up', 404);
-}
-
 if (!is_dir($backupDir)) {
     if (!mkdir($backupDir, 0755, true) && !is_dir($backupDir)) {
         json_error('SERVER_ERROR', 'Could not create backups directory', 500);
     }
 }
 
-$timestamp  = date('Y-m-d_H-i-s');
-$filename   = "questionnaire_database.$timestamp";
+$timestamp = date('Y-m-d_H-i-s');
+
+if (qdb_is_mysql()) {
+    [$host, $port] = [qdb_env_get('QDB_MYSQL_HOST') ?: '127.0.0.1', qdb_env_get('QDB_MYSQL_PORT') ?: '3306'];
+    [$user, $pass] = qdb_mysql_credentials();
+    $db = qdb_env_get('QDB_MYSQL_DATABASE') ?: 'nat-as-db';
+    $filename = "questionnaire_mysql_{$timestamp}.sql";
+    $backupFile = "$backupDir/$filename";
+    if (!function_exists('proc_open')) {
+        json_error('SERVER_ERROR', 'MySQL backup is unavailable: proc_open is disabled', 500);
+    }
+
+    $command = [
+        'mysqldump',
+        '--single-transaction',
+        '--routines',
+        '--triggers',
+        "--host={$host}",
+        "--port={$port}",
+        "--user={$user}",
+        $db,
+    ];
+    $environment = getenv();
+    if (!is_array($environment)) {
+        $environment = [];
+    }
+    if ($pass !== '') {
+        $environment['MYSQL_PWD'] = $pass;
+    }
+    $descriptors = [
+        0 => ['file', '/dev/null', 'r'],
+        1 => ['file', $backupFile, 'wb'],
+        2 => ['pipe', 'w'],
+    ];
+    $process = @proc_open($command, $descriptors, $pipes, null, $environment);
+    if (!is_resource($process)) {
+        @unlink($backupFile);
+        json_error('SERVER_ERROR', 'MySQL backup failed to start mysqldump', 500);
+    }
+    $errorOutput = stream_get_contents($pipes[2]);
+    fclose($pipes[2]);
+    $code = proc_close($process);
+    if ($code !== 0 || !is_file($backupFile) || filesize($backupFile) === 0) {
+        @unlink($backupFile);
+        $detail = trim((string)$errorOutput);
+        json_error('SERVER_ERROR', 'MySQL backup failed' . ($detail !== '' ? ": {$detail}" : ''), 500);
+    }
+    @chmod($backupFile, 0600);
+    json_success(['filename' => $filename]);
+}
+
+$dbPath = QDB_PATH;
+if (!file_exists($dbPath)) {
+    json_error('NOT_FOUND', 'No database to back up', 404);
+}
+
+$filename = "questionnaire_database.$timestamp";
 $backupFile = "$backupDir/$filename";
 
 if (!copy($dbPath, $backupFile)) {
diff --git a/handlers/questionnaires.php b/handlers/questionnaires.php
index 3a172a9..78a2e6c 100644
--- a/handlers/questionnaires.php
+++ b/handlers/questionnaires.php
@@ -107,9 +107,9 @@ case 'POST':
         $replaceIfExists = !empty($body['replaceIfExists']);
         [$pdo, $tmpDb, $lockFp] = qdb_open_write_or_fail();
         try {
-            $pdo->exec('PRAGMA foreign_keys = OFF;');
+            qdb_set_foreign_keys($pdo, false);
             $result = qdb_import_questionnaires_bundle($pdo, $bundle, $replaceIfExists);
-            $pdo->exec('PRAGMA foreign_keys = ON;');
+            qdb_set_foreign_keys($pdo, true);
             qdb_save($tmpDb, $lockFp);
             json_success($result);
         } catch (Throwable $e) {
@@ -208,7 +208,7 @@ case 'DELETE':
 
     [$pdo, $tmpDb, $lockFp] = qdb_open_write_or_fail();
     try {
-        $pdo->exec('PRAGMA foreign_keys = OFF;');
+        qdb_set_foreign_keys($pdo, false);
         $pdo->beginTransaction();
 
         $qIds = $pdo->prepare('SELECT questionID FROM question WHERE questionnaireID = :id');
@@ -231,7 +231,7 @@ case 'DELETE':
         $pdo->prepare('DELETE FROM questionnaire WHERE questionnaireID = :id')->execute([':id' => $id]);
 
         $pdo->commit();
-        $pdo->exec('PRAGMA foreign_keys = ON;');
+        qdb_set_foreign_keys($pdo, true);
         qdb_save($tmpDb, $lockFp);
         json_success([]);
     } catch (Throwable $e) {
diff --git a/handlers/questions.php b/handlers/questions.php
index 8bebf9d..28073ea 100644
--- a/handlers/questions.php
+++ b/handlers/questions.php
@@ -277,7 +277,7 @@ case 'DELETE':
             break;
         }
 
-        $pdo->exec('PRAGMA foreign_keys = OFF;');
+        qdb_set_foreign_keys($pdo, false);
         $aoIds = $pdo->prepare('SELECT answerOptionID FROM answer_option WHERE questionID = :id');
         $aoIds->execute([':id' => $id]);
         foreach ($aoIds->fetchAll(PDO::FETCH_COLUMN) as $aoid) {
@@ -287,7 +287,7 @@ case 'DELETE':
         $pdo->prepare('DELETE FROM question_score_rule WHERE questionID = :id')->execute([':id' => $id]);
         $pdo->prepare('DELETE FROM question_translation WHERE questionID = :id')->execute([':id' => $id]);
         $pdo->prepare('DELETE FROM question WHERE questionID = :id')->execute([':id' => $id]);
-        $pdo->exec('PRAGMA foreign_keys = ON;');
+        qdb_set_foreign_keys($pdo, true);
         $pdo->commit();
         qdb_save($tmpDb, $lockFp);
         json_success([
diff --git a/handlers/translations.php b/handlers/translations.php
index 3e06fa2..810525a 100644
--- a/handlers/translations.php
+++ b/handlers/translations.php
@@ -232,12 +232,12 @@ case 'PUT':
             if ($lang === QDB_SOURCE_LANGUAGE) {
                 qdb_ensure_source_language($pdo);
             } elseif (qdb_column_exists($pdo, 'language', 'enabled')) {
-                $pdo->prepare("INSERT INTO language (languageCode, name, enabled) VALUES (:lc, :n, 1)
-                               ON CONFLICT(languageCode) DO UPDATE SET name = excluded.name")
+                $pdo->prepare('INSERT INTO language (languageCode, name, enabled) VALUES (:lc, :n, 1) '
+                    . qdb_upsert_update(['languageCode'], ['name' => 'excluded.name', 'enabled' => '1']))
                     ->execute([':lc' => $lang, ':n' => $name]);
             } else {
-                $pdo->prepare("INSERT INTO language (languageCode, name) VALUES (:lc, :n)
-                               ON CONFLICT(languageCode) DO UPDATE SET name = excluded.name")
+                $pdo->prepare('INSERT INTO language (languageCode, name) VALUES (:lc, :n) '
+                    . qdb_upsert_update(['languageCode'], ['name' => 'excluded.name']))
                     ->execute([':lc' => $lang, ':n' => $name]);
             }
             qdb_save($tmpDb, $lockFp);
diff --git a/lib/analytics.php b/lib/analytics.php
index 1e69341..7faa5fd 100644
--- a/lib/analytics.php
+++ b/lib/analytics.php
@@ -255,11 +255,15 @@ function qdb_analytics_set_followup_note(PDO $pdo, array $tokenRec, string $clie
 
     $pdo->prepare(
         'INSERT INTO client_followup_note (clientCode, note, updatedByUserID, updatedAt)
-         VALUES (:cc, :n, :uid, :ts)
-         ON CONFLICT(clientCode) DO UPDATE SET
-            note = excluded.note,
-            updatedByUserID = excluded.updatedByUserID,
-            updatedAt = excluded.updatedAt'
+         VALUES (:cc, :n, :uid, :ts) '
+        . qdb_upsert_update(
+            ['clientCode'],
+            [
+                'note' => 'excluded.note',
+                'updatedByUserID' => 'excluded.updatedByUserID',
+                'updatedAt' => 'excluded.updatedAt',
+            ]
+        )
     )->execute([
         ':cc'  => $clientCode,
         ':n'   => $note,
diff --git a/lib/dev_fixture.php b/lib/dev_fixture.php
index 7b2ac5b..a693ff7 100644
--- a/lib/dev_fixture.php
+++ b/lib/dev_fixture.php
@@ -575,12 +575,16 @@ function qdb_dev_persist_submission(
     $answeredAt = $completedAt;
     $answerInsert = $pdo->prepare(
         'INSERT INTO client_answer (clientCode, questionID, answerOptionID, freeTextValue, numericValue, answeredAt)
-         VALUES (:cc, :qid, :aoid, :ftv, :nv, :at)
-         ON CONFLICT(clientCode, questionID) DO UPDATE SET
-             answerOptionID = excluded.answerOptionID,
-             freeTextValue  = excluded.freeTextValue,
-             numericValue   = excluded.numericValue,
-             answeredAt     = excluded.answeredAt'
+         VALUES (:cc, :qid, :aoid, :ftv, :nv, :at) '
+        . qdb_upsert_update(
+            ['clientCode', 'questionID'],
+            [
+                'answerOptionID' => 'excluded.answerOptionID',
+                'freeTextValue' => 'excluded.freeTextValue',
+                'numericValue' => 'excluded.numericValue',
+                'answeredAt' => 'excluded.answeredAt',
+            ]
+        )
     );
 
     foreach ($answers as $a) {
@@ -645,13 +649,17 @@ function qdb_dev_persist_submission(
 
     $pdo->prepare(
         'INSERT INTO completed_questionnaire (clientCode, questionnaireID, assignedByCoach, status, startedAt, completedAt, sumPoints)
-         VALUES (:cc, :qn, :abc, \'completed\', :sa, :ca, :sp)
-         ON CONFLICT(clientCode, questionnaireID) DO UPDATE SET
-             assignedByCoach = excluded.assignedByCoach,
-             status          = \'completed\',
-             startedAt       = COALESCE(excluded.startedAt, startedAt),
-             completedAt     = excluded.completedAt,
-             sumPoints       = excluded.sumPoints'
+         VALUES (:cc, :qn, :abc, \'completed\', :sa, :ca, :sp) '
+        . qdb_upsert_update(
+            ['clientCode', 'questionnaireID'],
+            [
+                'assignedByCoach' => 'excluded.assignedByCoach',
+                'status' => "'completed'",
+                'startedAt' => 'COALESCE(excluded.startedAt, startedAt)',
+                'completedAt' => 'excluded.completedAt',
+                'sumPoints' => 'excluded.sumPoints',
+            ]
+        )
     )->execute([
         ':cc'  => $clientCode,
         ':qn'  => $qnID,
@@ -863,7 +871,7 @@ function qdb_wipe_all_data_except_admins(PDO $pdo): array
     try {
         // Interview data references clients/coaches; coach references supervisor.
         // Disable FK checks for this bulk wipe (same pattern as questionnaire delete).
-        $pdo->exec('PRAGMA foreign_keys = OFF');
+        qdb_set_foreign_keys($pdo, false);
 
         $deleted['client_answers'] = (int)$pdo->exec('DELETE FROM client_answer');
         if (qdb_table_exists($pdo, 'client_answer_submission')) {
@@ -891,14 +899,14 @@ function qdb_wipe_all_data_except_admins(PDO $pdo): array
         $users->execute();
         $deleted['users'] = $users->rowCount();
 
-        $pdo->exec('PRAGMA foreign_keys = ON');
+        qdb_set_foreign_keys($pdo, true);
         $pdo->commit();
     } catch (Throwable $e) {
         if ($pdo->inTransaction()) {
             $pdo->rollBack();
         }
         try {
-            $pdo->exec('PRAGMA foreign_keys = ON');
+            qdb_set_foreign_keys($pdo, true);
         } catch (Throwable $ignored) {
         }
         throw $e;
diff --git a/lib/questionnaire_structure.php b/lib/questionnaire_structure.php
index a4e6e18..991f3ee 100644
--- a/lib/questionnaire_structure.php
+++ b/lib/questionnaire_structure.php
@@ -104,10 +104,14 @@ function qdb_save_structure_snapshot(PDO $pdo, string $qnID, int $revision, arra
     }
     $pdo->prepare(
         'INSERT INTO questionnaire_structure_snapshot (questionnaireID, structureRevision, createdAt, manifestJson)
-         VALUES (:qn, :rev, :ts, :mj)
-         ON CONFLICT(questionnaireID, structureRevision) DO UPDATE SET
-            manifestJson = excluded.manifestJson,
-            createdAt = excluded.createdAt'
+         VALUES (:qn, :rev, :ts, :mj) '
+        . qdb_upsert_update(
+            ['questionnaireID', 'structureRevision'],
+            [
+                'manifestJson' => 'excluded.manifestJson',
+                'createdAt' => 'excluded.createdAt',
+            ]
+        )
     )->execute([
         ':qn'  => $qnID,
         ':rev' => $revision,
@@ -168,10 +172,7 @@ function qdb_option_has_client_data(PDO $pdo, string $answerOptionID): bool {
     if ($stmt->fetchColumn()) {
         return true;
     }
-    $chk = $pdo->query(
-        "SELECT 1 FROM sqlite_master WHERE type = 'table' AND name = 'client_answer_submission' LIMIT 1"
-    );
-    if (!$chk || !$chk->fetchColumn()) {
+    if (!qdb_table_exists($pdo, 'client_answer_submission')) {
         return false;
     }
     $stmt = $pdo->prepare(
@@ -187,10 +188,7 @@ function qdb_question_has_client_data(PDO $pdo, string $questionID): bool {
     if ($stmt->fetchColumn()) {
         return true;
     }
-    $chk = $pdo->query(
-        "SELECT 1 FROM sqlite_master WHERE type = 'table' AND name = 'client_answer_submission' LIMIT 1"
-    );
-    if (!$chk || !$chk->fetchColumn()) {
+    if (!qdb_table_exists($pdo, 'client_answer_submission')) {
         return false;
     }
     $stmt = $pdo->prepare(
diff --git a/lib/read_db_cache.php b/lib/read_db_cache.php
index 6f58a99..0283147 100644
--- a/lib/read_db_cache.php
+++ b/lib/read_db_cache.php
@@ -12,6 +12,10 @@ define('QDB_READ_GENERATION_FILE', QDB_UPLOADS_DIR . '/.qdb_read_generation');
  * @return array{0: PDO, 1: string, 2: null}
  */
 function qdb_read_db_open(): array {
+    if (qdb_is_mysql()) {
+        return qdb_mysql_open();
+    }
+
     $signature = qdb_read_cache_signature();
     $cached = qdb_read_cache_get($signature);
     if ($cached !== null) {
diff --git a/lib/scoring.php b/lib/scoring.php
index db58c25..a96773d 100644
--- a/lib/scoring.php
+++ b/lib/scoring.php
@@ -383,12 +383,16 @@ function qdb_recompute_profile_scores_for_client(PDO $pdo, string $clientCode, ?
     $upsert = $pdo->prepare(
         'INSERT INTO client_scoring_profile_result
             (clientCode, profileID, weightedTotal, band, computedAt, questionnaireSnapshot)
-         VALUES (:cc, :pid, :wt, :band, :at, :snap)
-         ON CONFLICT(clientCode, profileID) DO UPDATE SET
-            weightedTotal = excluded.weightedTotal,
-            band = excluded.band,
-            computedAt = excluded.computedAt,
-            questionnaireSnapshot = excluded.questionnaireSnapshot'
+         VALUES (:cc, :pid, :wt, :band, :at, :snap) '
+        . qdb_upsert_update(
+            ['clientCode', 'profileID'],
+            [
+                'weightedTotal' => 'excluded.weightedTotal',
+                'band' => 'excluded.band',
+                'computedAt' => 'excluded.computedAt',
+                'questionnaireSnapshot' => 'excluded.questionnaireSnapshot',
+            ]
+        )
     );
 
     foreach ($profiles as $profile) {
diff --git a/lib/settings.php b/lib/settings.php
index 6d644d6..6dac954 100644
--- a/lib/settings.php
+++ b/lib/settings.php
@@ -97,8 +97,8 @@ function qdb_settings_save_on_pdo(PDO $pdo, array $settings): void
     qdb_ensure_system_setting_table($pdo);
     $stmt = $pdo->prepare(
         'INSERT INTO system_setting (settingKey, settingValue)
-         VALUES (:k, :v)
-         ON CONFLICT(settingKey) DO UPDATE SET settingValue = excluded.settingValue'
+         VALUES (:k, :v) '
+        . qdb_upsert_update(['settingKey'], ['settingValue' => 'excluded.settingValue'])
     );
     foreach (qdb_settings_keys() as $key) {
         $stmt->execute([':k' => $key, ':v' => (string)(int)($settings[$key] ?? 0)]);
diff --git a/lib/sql_dialect.php b/lib/sql_dialect.php
new file mode 100644
index 0000000..38de566
--- /dev/null
+++ b/lib/sql_dialect.php
@@ -0,0 +1,226 @@
+<?php
+
+/** Database driver helpers (SQLite legacy + MySQL). */
+
+function qdb_driver(): string {
+    if (isset($GLOBALS['qdb_driver_cache'])) {
+        return (string)$GLOBALS['qdb_driver_cache'];
+    }
+    $GLOBALS['qdb_driver_cache'] = qdb_driver_resolve();
+    return (string)$GLOBALS['qdb_driver_cache'];
+}
+
+function qdb_reset_driver_cache(): void {
+    unset($GLOBALS['qdb_driver_cache']);
+}
+
+function qdb_driver_resolve(): string {
+    $configured = strtolower(trim((string)(qdb_env_get('QDB_DRIVER') ?? '')));
+    if ($configured === 'mysql' || $configured === 'sqlite') {
+        $driver = $configured;
+        return $driver;
+    }
+    if (qdb_env_get('QDB_MYSQL_HOST') || qdb_env_get('QDB_MYSQL_DATABASE')) {
+        $driver = 'mysql';
+        return $driver;
+    }
+    $driver = 'sqlite';
+    return $driver;
+}
+
+function qdb_is_mysql(): bool {
+    return qdb_driver() === 'mysql';
+}
+
+function qdb_is_sqlite(): bool {
+    return qdb_driver() === 'sqlite';
+}
+
+function qdb_schema_file(): string {
+    return qdb_is_mysql()
+        ? __DIR__ . '/../schema.mysql.sql'
+        : QDB_SCHEMA;
+}
+
+function qdb_mysql_dsn(): string {
+    $host = qdb_env_get('QDB_MYSQL_HOST') ?: '127.0.0.1';
+    $port = qdb_env_get('QDB_MYSQL_PORT') ?: '3306';
+    $db = qdb_env_get('QDB_MYSQL_DATABASE') ?: 'nat-as-db';
+    return "mysql:host={$host};port={$port};dbname={$db};charset=utf8mb4";
+}
+
+function qdb_mysql_credentials(): array {
+    return [
+        qdb_env_get('QDB_MYSQL_USER') ?: 'nat_as_db',
+        qdb_env_get('QDB_MYSQL_PASSWORD') ?: '',
+    ];
+}
+
+/** @return array{0: PDO, 1: string, 2: null} */
+function qdb_mysql_open(): array {
+    static $pdo = null;
+    if ($pdo === null) {
+        [$user, $pass] = qdb_mysql_credentials();
+        $pdo = new PDO(qdb_mysql_dsn(), $user, $pass, [
+            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
+            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
+        ]);
+    }
+    return [$pdo, '', null];
+}
+
+function qdb_set_foreign_keys(PDO $pdo, bool $enabled): void {
+    if (qdb_is_mysql()) {
+        $pdo->exec('SET FOREIGN_KEY_CHECKS = ' . ($enabled ? '1' : '0'));
+        return;
+    }
+    $pdo->exec('PRAGMA foreign_keys = ' . ($enabled ? 'ON' : 'OFF') . ';');
+}
+
+function qdb_table_exists(PDO $pdo, string $table): bool {
+    if (qdb_is_mysql()) {
+        $stmt = $pdo->prepare(
+            'SELECT 1 FROM information_schema.tables
+             WHERE table_schema = DATABASE() AND table_name = :t LIMIT 1'
+        );
+        $stmt->execute([':t' => $table]);
+        return (bool)$stmt->fetchColumn();
+    }
+    $stmt = $pdo->prepare(
+        "SELECT 1 FROM sqlite_master WHERE type = 'table' AND name = :t LIMIT 1"
+    );
+    $stmt->execute([':t' => $table]);
+    return (bool)$stmt->fetchColumn();
+}
+
+function qdb_column_exists(PDO $pdo, string $table, string $column): bool {
+    if (qdb_is_mysql()) {
+        $stmt = $pdo->prepare(
+            'SELECT 1 FROM information_schema.columns
+             WHERE table_schema = DATABASE() AND table_name = :t AND column_name = :c LIMIT 1'
+        );
+        $stmt->execute([':t' => $table, ':c' => $column]);
+        return (bool)$stmt->fetchColumn();
+    }
+    $stmt = $pdo->query('PRAGMA table_info(' . $table . ')');
+    foreach ($stmt->fetchAll(PDO::FETCH_ASSOC) as $col) {
+        if (($col['name'] ?? '') === $column) {
+            return true;
+        }
+    }
+    return false;
+}
+
+function qdb_schema_version(PDO $pdo): int {
+    if (qdb_is_mysql()) {
+        if (!qdb_table_exists($pdo, 'schema_version')) {
+            return 0;
+        }
+        return (int)$pdo->query('SELECT version FROM schema_version WHERE id = 1')->fetchColumn();
+    }
+    return (int)$pdo->query('PRAGMA user_version')->fetchColumn();
+}
+
+function qdb_set_schema_version(PDO $pdo, int $version): void {
+    if (qdb_is_mysql()) {
+        if (!qdb_table_exists($pdo, 'schema_version')) {
+            $pdo->exec(
+                'CREATE TABLE IF NOT EXISTS schema_version (
+                    id INT NOT NULL PRIMARY KEY,
+                    version INT NOT NULL
+                ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4'
+            );
+        }
+        $pdo->prepare(
+            'INSERT INTO schema_version (id, version) VALUES (1, :v)
+             ON DUPLICATE KEY UPDATE version = VALUES(version)'
+        )->execute([':v' => $version]);
+        return;
+    }
+    $pdo->exec('PRAGMA user_version = ' . (int)$version . ';');
+}
+
+function qdb_stmt_affected_rows(PDO $pdo, PDOStatement $stmt): int {
+    if (qdb_is_mysql()) {
+        return $stmt->rowCount();
+    }
+    return (int)$pdo->query('SELECT changes()')->fetchColumn();
+}
+
+function qdb_create_index_if_not_exists(PDO $pdo, string $name, string $table, string $columnsSql): void {
+    if (qdb_is_mysql()) {
+        if (!qdb_table_exists($pdo, $table)) {
+            return;
+        }
+        $stmt = $pdo->prepare(
+            'SELECT 1 FROM information_schema.statistics
+             WHERE table_schema = DATABASE() AND table_name = :t AND index_name = :i LIMIT 1'
+        );
+        $stmt->execute([':t' => $table, ':i' => $name]);
+        if ($stmt->fetchColumn()) {
+            return;
+        }
+        $pdo->exec("CREATE INDEX `{$name}` ON `{$table}` ({$columnsSql})");
+        return;
+    }
+    $pdo->exec("CREATE INDEX IF NOT EXISTS {$name} ON {$table}({$columnsSql})");
+}
+
+/**
+ * @param list<string> $conflictColumns
+ */
+function qdb_upsert_do_nothing(array $conflictColumns): string {
+    if (qdb_is_mysql()) {
+        $first = $conflictColumns[0];
+        return "ON DUPLICATE KEY UPDATE `{$first}` = `{$first}`";
+    }
+    return 'ON CONFLICT(' . implode(', ', $conflictColumns) . ') DO NOTHING';
+}
+
+/**
+ * @param list<string> $conflictColumns
+ * @param array<string, string> $assignments column => expression (use excluded.col on SQLite)
+ */
+function qdb_upsert_update(array $conflictColumns, array $assignments): string {
+    if (qdb_is_mysql()) {
+        $parts = [];
+        foreach ($assignments as $column => $expression) {
+            $mysqlExpr = preg_replace('/\bexcluded\.(\w+)\b/', 'VALUES($1)', $expression) ?? $expression;
+            $parts[] = "`{$column}` = {$mysqlExpr}";
+        }
+        return 'ON DUPLICATE KEY UPDATE ' . implode(', ', $parts);
+    }
+    $parts = [];
+    foreach ($assignments as $column => $expression) {
+        $parts[] = "{$column} = {$expression}";
+    }
+    return 'ON CONFLICT(' . implode(', ', $conflictColumns) . ') DO UPDATE SET ' . implode(', ', $parts);
+}
+
+function qdb_exec_schema_file(PDO $pdo, string $schemaPath): void {
+    $sql = file_get_contents($schemaPath);
+    if ($sql === false) {
+        throw new RuntimeException("Could not read schema: {$schemaPath}");
+    }
+    qdb_exec_schema_sql($pdo, $sql);
+}
+
+function qdb_exec_schema_sql(PDO $pdo, string $sql): void {
+    if (qdb_is_mysql()) {
+        foreach (qdb_split_sql_statements($sql) as $statement) {
+            $trimmed = trim($statement);
+            if ($trimmed === '' || str_starts_with($trimmed, '--')) {
+                continue;
+            }
+            $pdo->exec($trimmed);
+        }
+        return;
+    }
+    $pdo->exec($sql);
+}
+
+/** @return list<string> */
+function qdb_split_sql_statements(string $sql): array {
+    $parts = preg_split('/;\s*\n/', $sql) ?: [];
+    return array_values(array_filter(array_map('trim', $parts), fn($s) => $s !== ''));
+}
diff --git a/schema.mysql.sql b/schema.mysql.sql
new file mode 100644
index 0000000..e0e2740
--- /dev/null
+++ b/schema.mysql.sql
@@ -0,0 +1,270 @@
+CREATE TABLE IF NOT EXISTS schema_version (
+    id      INT NOT NULL PRIMARY KEY,
+    version INT NOT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS users (
+    userID             VARCHAR(64)  NOT NULL PRIMARY KEY,
+    username           VARCHAR(191) NOT NULL UNIQUE,
+    passwordHash       VARCHAR(255) NOT NULL,
+    role               VARCHAR(32)  NOT NULL,
+    entityID           VARCHAR(64)  NOT NULL,
+    mustChangePassword TINYINT(1)   NOT NULL DEFAULT 1,
+    createdAt          BIGINT       NOT NULL,
+    CONSTRAINT chk_users_role CHECK (role IN ('admin','supervisor','coach'))
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS admin (
+    adminID  VARCHAR(64)  NOT NULL PRIMARY KEY,
+    username VARCHAR(191) NOT NULL,
+    location VARCHAR(255) NOT NULL DEFAULT ''
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS supervisor (
+    supervisorID VARCHAR(64)  NOT NULL PRIMARY KEY,
+    username     VARCHAR(191) NOT NULL,
+    location     VARCHAR(255) NOT NULL DEFAULT ''
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS coach (
+    coachID      VARCHAR(64) NOT NULL PRIMARY KEY,
+    supervisorID VARCHAR(64) NOT NULL,
+    username     VARCHAR(191) NOT NULL,
+    FOREIGN KEY(supervisorID) REFERENCES supervisor(supervisorID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS client (
+    clientCode VARCHAR(191) NOT NULL PRIMARY KEY,
+    coachID    VARCHAR(64)  NOT NULL,
+    archived   TINYINT(1)   NOT NULL DEFAULT 0,
+    archivedAt BIGINT       NOT NULL DEFAULT 0,
+    FOREIGN KEY(coachID) REFERENCES coach(coachID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS questionnaire_category (
+    categoryKey VARCHAR(191) NOT NULL PRIMARY KEY,
+    label       VARCHAR(255) NOT NULL DEFAULT '',
+    color       VARCHAR(32)  NOT NULL DEFAULT '#64748b',
+    orderIndex  INT          NOT NULL DEFAULT 0
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS questionnaire (
+    questionnaireID    VARCHAR(191) NOT NULL PRIMARY KEY,
+    name               VARCHAR(255) NOT NULL DEFAULT '',
+    version            VARCHAR(64)  NOT NULL DEFAULT '',
+    state              VARCHAR(64)  NOT NULL DEFAULT '',
+    orderIndex         INT          NOT NULL DEFAULT 0,
+    showPoints         TINYINT(1)   NOT NULL DEFAULT 0,
+    conditionJson      LONGTEXT     NOT NULL,
+    categoryKey        VARCHAR(191) NOT NULL DEFAULT '',
+    structureRevision  INT          NOT NULL DEFAULT 1,
+    structureChangedAt BIGINT       NOT NULL DEFAULT 0
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS questionnaire_structure_snapshot (
+    questionnaireID   VARCHAR(191) NOT NULL,
+    structureRevision INT          NOT NULL,
+    createdAt         BIGINT       NOT NULL,
+    manifestJson      LONGTEXT     NOT NULL,
+    PRIMARY KEY (questionnaireID, structureRevision),
+    FOREIGN KEY(questionnaireID) REFERENCES questionnaire(questionnaireID) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS question (
+    questionID        VARCHAR(64)  NOT NULL PRIMARY KEY,
+    questionnaireID   VARCHAR(191) NOT NULL,
+    defaultText       LONGTEXT     NOT NULL,
+    type              VARCHAR(64)  NOT NULL DEFAULT '',
+    orderIndex        INT          NOT NULL DEFAULT 0,
+    isRequired        TINYINT(1)   NOT NULL DEFAULT 0,
+    configJson        LONGTEXT     NOT NULL,
+    retiredAt         BIGINT       NOT NULL DEFAULT 0,
+    retiredInRevision INT          NOT NULL DEFAULT 0,
+    FOREIGN KEY(questionnaireID) REFERENCES questionnaire(questionnaireID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS answer_option (
+    answerOptionID VARCHAR(64) NOT NULL PRIMARY KEY,
+    questionID     VARCHAR(64) NOT NULL,
+    defaultText    LONGTEXT    NOT NULL,
+    points         INT         NOT NULL DEFAULT 0,
+    orderIndex     INT         NOT NULL DEFAULT 0,
+    nextQuestionId VARCHAR(64) NOT NULL DEFAULT '',
+    retiredAt      BIGINT      NOT NULL DEFAULT 0,
+    FOREIGN KEY(questionID) REFERENCES question(questionID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS client_answer (
+    clientCode     VARCHAR(191) NOT NULL,
+    questionID     VARCHAR(64)  NOT NULL,
+    answerOptionID VARCHAR(64)  NULL,
+    freeTextValue  LONGTEXT     NULL,
+    numericValue   DOUBLE       NULL,
+    answeredAt     BIGINT       NULL,
+    PRIMARY KEY (clientCode, questionID),
+    FOREIGN KEY(clientCode) REFERENCES client(clientCode),
+    FOREIGN KEY(questionID) REFERENCES question(questionID),
+    FOREIGN KEY(answerOptionID) REFERENCES answer_option(answerOptionID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS completed_questionnaire (
+    clientCode      VARCHAR(191) NOT NULL,
+    questionnaireID VARCHAR(191) NOT NULL,
+    assignedByCoach VARCHAR(64)  NULL,
+    status          VARCHAR(64)  NOT NULL DEFAULT '',
+    startedAt       BIGINT       NULL,
+    completedAt     BIGINT       NULL,
+    sumPoints       INT          NULL,
+    PRIMARY KEY (clientCode, questionnaireID),
+    FOREIGN KEY(clientCode) REFERENCES client(clientCode),
+    FOREIGN KEY(questionnaireID) REFERENCES questionnaire(questionnaireID),
+    FOREIGN KEY(assignedByCoach) REFERENCES coach(coachID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS question_translation (
+    questionID   VARCHAR(64) NOT NULL,
+    languageCode VARCHAR(16) NOT NULL,
+    text         LONGTEXT    NOT NULL,
+    PRIMARY KEY (questionID, languageCode),
+    FOREIGN KEY(questionID) REFERENCES question(questionID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS answer_option_translation (
+    answerOptionID VARCHAR(64) NOT NULL,
+    languageCode   VARCHAR(16) NOT NULL,
+    text           LONGTEXT    NOT NULL,
+    PRIMARY KEY (answerOptionID, languageCode),
+    FOREIGN KEY(answerOptionID) REFERENCES answer_option(answerOptionID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS string_translation (
+    stringKey    VARCHAR(191) NOT NULL,
+    languageCode VARCHAR(16)  NOT NULL,
+    text         LONGTEXT     NOT NULL,
+    PRIMARY KEY (stringKey, languageCode)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS language (
+    languageCode VARCHAR(16)  NOT NULL PRIMARY KEY,
+    name         VARCHAR(191) NOT NULL DEFAULT '',
+    enabled      TINYINT(1)   NOT NULL DEFAULT 1
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS questionnaire_language_disable (
+    questionnaireID VARCHAR(191) NOT NULL,
+    languageCode    VARCHAR(16)  NOT NULL,
+    PRIMARY KEY (questionnaireID, languageCode),
+    FOREIGN KEY(questionnaireID) REFERENCES questionnaire(questionnaireID) ON DELETE CASCADE,
+    FOREIGN KEY(languageCode) REFERENCES language(languageCode) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS session (
+    token     VARCHAR(128) NOT NULL PRIMARY KEY,
+    userID    VARCHAR(64)  NOT NULL,
+    role      VARCHAR(32)  NOT NULL,
+    entityID  VARCHAR(64)  NOT NULL DEFAULT '',
+    createdAt BIGINT       NOT NULL,
+    expiresAt BIGINT       NOT NULL,
+    temp      TINYINT(1)   NOT NULL DEFAULT 0
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS questionnaire_submission (
+    submissionID          VARCHAR(64)  NOT NULL PRIMARY KEY,
+    clientCode            VARCHAR(191) NOT NULL,
+    questionnaireID     VARCHAR(191) NOT NULL,
+    version               INT          NOT NULL,
+    submittedAt           BIGINT       NOT NULL,
+    submittedByUserID     VARCHAR(64)  NOT NULL DEFAULT '',
+    submittedByRole       VARCHAR(32)  NOT NULL DEFAULT '',
+    assignedByCoach       VARCHAR(64)  NULL,
+    status                VARCHAR(64)  NOT NULL DEFAULT '',
+    startedAt             BIGINT       NULL,
+    completedAt           BIGINT       NULL,
+    sumPoints             INT          NULL,
+    structureRevision     INT          NOT NULL DEFAULT 1,
+    structureSnapshotJson LONGTEXT     NOT NULL,
+    FOREIGN KEY(clientCode) REFERENCES client(clientCode),
+    FOREIGN KEY(questionnaireID) REFERENCES questionnaire(questionnaireID),
+    FOREIGN KEY(assignedByCoach) REFERENCES coach(coachID),
+    UNIQUE KEY uq_submission_client_qn_version (clientCode, questionnaireID, version),
+    KEY idx_submission_client_qn (clientCode, questionnaireID),
+    KEY idx_submission_client_time (clientCode, submittedAt)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS client_answer_submission (
+    submissionID   VARCHAR(64) NOT NULL,
+    questionID     VARCHAR(64) NOT NULL,
+    answerOptionID VARCHAR(64) NULL,
+    freeTextValue  LONGTEXT    NULL,
+    numericValue   DOUBLE      NULL,
+    answeredAt     BIGINT      NULL,
+    PRIMARY KEY (submissionID, questionID),
+    FOREIGN KEY(submissionID) REFERENCES questionnaire_submission(submissionID) ON DELETE CASCADE,
+    FOREIGN KEY(questionID) REFERENCES question(questionID),
+    FOREIGN KEY(answerOptionID) REFERENCES answer_option(answerOptionID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS client_followup_note (
+    clientCode      VARCHAR(191) NOT NULL PRIMARY KEY,
+    note            LONGTEXT     NOT NULL,
+    updatedByUserID VARCHAR(64)  NOT NULL DEFAULT '',
+    updatedAt       BIGINT       NOT NULL,
+    FOREIGN KEY(clientCode) REFERENCES client(clientCode)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS system_setting (
+    settingKey   VARCHAR(191) NOT NULL PRIMARY KEY,
+    settingValue LONGTEXT     NOT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS question_score_rule (
+    ruleID     VARCHAR(64) NOT NULL PRIMARY KEY,
+    questionID VARCHAR(64) NOT NULL,
+    scopeKey   VARCHAR(191) NOT NULL DEFAULT '',
+    levelKey   VARCHAR(191) NOT NULL,
+    points     INT         NOT NULL DEFAULT 0,
+    FOREIGN KEY(questionID) REFERENCES question(questionID) ON DELETE CASCADE,
+    UNIQUE KEY uq_score_rule_question_scope_level (questionID, scopeKey, levelKey),
+    KEY idx_score_rule_question (questionID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS scoring_profile (
+    profileID            VARCHAR(64)  NOT NULL PRIMARY KEY,
+    name                 VARCHAR(255) NOT NULL DEFAULT '',
+    description          LONGTEXT     NOT NULL,
+    isActive             TINYINT(1)   NOT NULL DEFAULT 1,
+    greenMin             INT          NOT NULL DEFAULT 0,
+    greenMax             INT          NOT NULL DEFAULT 12,
+    yellowMin            INT          NOT NULL DEFAULT 13,
+    yellowMax            INT          NOT NULL DEFAULT 36,
+    redMin               INT          NOT NULL DEFAULT 37,
+    processCompleteBands LONGTEXT     NOT NULL,
+    createdAt            BIGINT       NOT NULL DEFAULT 0,
+    updatedAt            BIGINT       NOT NULL DEFAULT 0
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS scoring_profile_questionnaire (
+    profileID       VARCHAR(64)  NOT NULL,
+    questionnaireID VARCHAR(191) NOT NULL,
+    weight          DOUBLE       NOT NULL DEFAULT 1.0,
+    orderIndex      INT          NOT NULL DEFAULT 0,
+    PRIMARY KEY (profileID, questionnaireID),
+    FOREIGN KEY(profileID) REFERENCES scoring_profile(profileID) ON DELETE CASCADE,
+    FOREIGN KEY(questionnaireID) REFERENCES questionnaire(questionnaireID) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS client_scoring_profile_result (
+    clientCode            VARCHAR(191) NOT NULL,
+    profileID             VARCHAR(64)  NOT NULL,
+    weightedTotal         DOUBLE       NOT NULL DEFAULT 0,
+    band                  VARCHAR(32)  NOT NULL DEFAULT '',
+    computedAt            BIGINT       NOT NULL DEFAULT 0,
+    questionnaireSnapshot LONGTEXT     NOT NULL,
+    coachBand             VARCHAR(32)  NOT NULL DEFAULT '',
+    coachReviewedAt       BIGINT       NOT NULL DEFAULT 0,
+    coachReviewedByUserID VARCHAR(64)  NOT NULL DEFAULT '',
+    PRIMARY KEY (clientCode, profileID),
+    FOREIGN KEY(clientCode) REFERENCES client(clientCode) ON DELETE CASCADE,
+    FOREIGN KEY(profileID) REFERENCES scoring_profile(profileID) ON DELETE CASCADE,
+    KEY idx_client_profile_result_profile (profileID)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
diff --git a/tests/Integration/DbInitTest.php b/tests/Integration/DbInitTest.php
index 965225b..fdf74dd 100644
--- a/tests/Integration/DbInitTest.php
+++ b/tests/Integration/DbInitTest.php
@@ -16,8 +16,8 @@ final class DbInitTest extends QdbTestCase
         qdb_discard($t1, $l1);
 
         [$pdo2, $t2, $l2] = qdb_open(true);
-        $pdo2->exec("INSERT INTO language (languageCode, name) VALUES ('fr', 'French')
-                     ON CONFLICT(languageCode) DO NOTHING");
+        $pdo2->exec("INSERT INTO language (languageCode, name) VALUES ('fr', 'French') "
+            . qdb_upsert_do_nothing(['languageCode']));
         qdb_save($t2, $l2);
 
         [$pdo3, $t3, $l3] = qdb_open(false);
@@ -35,6 +35,26 @@ final class DbInitTest extends QdbTestCase
         qdb_discard($tmp, $lock);
     }
 
+    public function testSchemaSqlContentIsExecutedDirectly(): void
+    {
+        $pdo = new \PDO('sqlite::memory:');
+        $pdo->setAttribute(\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
+
+        qdb_exec_schema_sql($pdo, 'CREATE TABLE schema_sql_test (id INTEGER PRIMARY KEY);');
+
+        $this->assertTrue(qdb_table_exists($pdo, 'schema_sql_test'));
+    }
+
+    public function testRuntimeDriverOverrideWinsOverConfiguration(): void
+    {
+        qdb_env_override('QDB_DRIVER', 'mysql');
+        $this->assertSame('mysql', qdb_env_get('QDB_DRIVER'));
+        $this->assertTrue(qdb_is_mysql());
+
+        qdb_env_override('QDB_DRIVER', 'sqlite');
+        $this->assertTrue(qdb_is_sqlite());
+    }
+
     public function testMigrationsRecreateDroppedSubmissionTables(): void
     {
         [$pdo, $tmp, $lock] = qdb_open(true);
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 8c10c6d..1ef0891 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -22,8 +22,9 @@ require dirname(__DIR__) . '/vendor/autoload.php';
 require_once dirname(__DIR__) . '/lib/response.php';
 require_once dirname(__DIR__) . '/common.php';
 
+qdb_env_override('QDB_DRIVER', 'sqlite');
+
 // Always use the test key, even when .env defines QDB_MASTER_KEY (avoids decrypt/seed drift).
-qdb_env_set('QDB_MASTER_KEY', $testMasterKey);
-$GLOBALS['qdb_dotenv']['QDB_MASTER_KEY'] = $testMasterKey;
+qdb_env_override('QDB_MASTER_KEY', $testMasterKey);
 
 require_once dirname(__DIR__) . '/db_init.php';
diff --git a/tests/mysql_smoke.php b/tests/mysql_smoke.php
new file mode 100644
index 0000000..1749eb0
--- /dev/null
+++ b/tests/mysql_smoke.php
@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+require dirname(__DIR__) . '/vendor/autoload.php';
+require_once dirname(__DIR__) . '/lib/response.php';
+require_once dirname(__DIR__) . '/common.php';
+
+qdb_env_override('QDB_DRIVER', 'mysql');
+require_once dirname(__DIR__) . '/db_init.php';
+
+[$pdo] = qdb_open(true);
+
+foreach (['users', 'questionnaire', 'questionnaire_submission', 'system_setting'] as $table) {
+    if (!qdb_table_exists($pdo, $table)) {
+        throw new RuntimeException("MySQL smoke test: missing table {$table}");
+    }
+}
+
+if (qdb_schema_version($pdo) !== QDB_VERSION) {
+    throw new RuntimeException('MySQL smoke test: incorrect schema version');
+}
+
+// Schema application must be safe to retry after an interrupted or concurrent bootstrap.
+qdb_exec_schema_file($pdo, qdb_schema_file());
+
+$pdo->exec("DELETE FROM language WHERE languageCode = 'ci-smoke'");
+$upsert = $pdo->prepare(
+    'INSERT INTO language (languageCode, name, enabled) VALUES (:code, :name, 1) '
+    . qdb_upsert_update(['languageCode'], ['name' => 'excluded.name'])
+);
+$upsert->execute([':code' => 'ci-smoke', ':name' => 'First']);
+$upsert->execute([':code' => 'ci-smoke', ':name' => 'Updated']);
+$name = $pdo->query("SELECT name FROM language WHERE languageCode = 'ci-smoke'")->fetchColumn();
+if ($name !== 'Updated') {
+    throw new RuntimeException('MySQL smoke test: upsert failed');
+}
+$pdo->exec("DELETE FROM language WHERE languageCode = 'ci-smoke'");
+
+fwrite(STDOUT, "MySQL smoke test passed\n");