WordPress Custom Database Tables

Model note: dbDelta schema and CRUD patterns are mechanical (haiku). Query optimisation and multi-version data migrations require cross-file reasoning — use sonnet for those sub-tasks.

Create and manage custom database tables in WordPress plugins: dbDelta() for schema definition, versioned upgrade routines, $wpdb CRUD with prepared statements, and data migration strategies.

When to use

• "Create a custom DB table for my plugin", "set up plugin schema with dbDelta".
• "Write a migration for plugin upgrade", "run schema changes on update".
• "Query a custom table", "insert/update/delete with $wpdb".
• "Optimise a slow custom query", "add an index to a plugin table".
• "Migrate data from post meta to a custom table".

Not for: WooCommerce order table operations — use wp-woocommerce. General $wpdb query optimisation in core WP tables — use wp-performance (official skill).

Method

1. Create table with dbDelta

dbDelta() is the only WP-safe way to create and alter tables — it diffs the current schema against the SQL and applies only the necessary changes.

function my_plugin_create_tables() {
    global $wpdb;
    $charset_collate = $wpdb->get_charset_collate(); // e.g. DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_520_ci

    // $wpdb->prefix respects multisite site prefix automatically
    $table_log  = $wpdb->prefix . 'my_plugin_log';
    $table_meta = $wpdb->prefix . 'my_plugin_item_meta';

    // IMPORTANT: two spaces before PRIMARY KEY, one space before each KEY
    // IMPORTANT: no trailing comma on last field before closing paren
    $sql = "CREATE TABLE {$table_log} (
  id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  item_id bigint(20) unsigned NOT NULL,
  action varchar(100) NOT NULL DEFAULT '',
  message longtext NOT NULL,
  user_id bigint(20) unsigned NOT NULL DEFAULT 0,
  created_at datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (id),
  KEY item_id (item_id),
  KEY created_at (created_at)
) {$charset_collate};

CREATE TABLE {$table_meta} (
  meta_id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  item_id bigint(20) unsigned NOT NULL,
  meta_key varchar(255) NOT NULL DEFAULT '',
  meta_value longtext,
  PRIMARY KEY  (meta_id),
  KEY item_id (item_id),
  KEY meta_key (meta_key(191))
) {$charset_collate};";

    require_once ABSPATH . 'wp-admin/includes/upgrade.php';
    dbDelta( $sql );
}

Critical dbDelta formatting rules (violations cause silent failures):

• Two spaces after PRIMARY KEY (e.g. PRIMARY KEY (id))
• Field definitions must end with a comma except the last field before the closing paren
• KEY lines go after all field definitions, before the closing paren
• Only CREATE TABLE statements — no ALTER TABLE (dbDelta handles column additions, not removals)
• Always include {$charset_collate} at the end

2. Versioned upgrade routine

Track schema version in an option; only re-run dbDelta when the version changes:

define( 'MY_PLUGIN_DB_VERSION', '1.3.0' );

function my_plugin_maybe_upgrade_db() {
    $installed = get_option( 'my_plugin_db_version', '0' );
    if ( version_compare( $installed, MY_PLUGIN_DB_VERSION, '>=' ) ) {
        return; // already up to date
    }

    my_plugin_create_tables(); // always safe to re-run dbDelta

    // Version-specific data migrations
    if ( version_compare( $installed, '1.2.0', '<' ) ) {
        my_plugin_migrate_to_1_2_0();
    }
    if ( version_compare( $installed, '1.3.0', '<' ) ) {
        my_plugin_migrate_to_1_3_0();
    }

    update_option( 'my_plugin_db_version', MY_PLUGIN_DB_VERSION );
}
add_action( 'plugins_loaded', 'my_plugin_maybe_upgrade_db' );

Run on plugins_loaded (every request until updated), not just on activation — catches updates after auto-update or version switch.

3. $wpdb CRUD

Always use $wpdb->prepare() for any value from user input or untrusted source.

Insert:

$result = $wpdb->insert(
    $wpdb->prefix . 'my_plugin_log',
    [
        'item_id'    => $item_id,
        'action'     => 'view',
        'message'    => $message,
        'user_id'    => get_current_user_id(),
        'created_at' => current_time( 'mysql' ),
    ],
    [ '%d', '%s', '%s', '%d', '%s' ] // format for each value: %d int, %s string, %f float
);
$inserted_id = $wpdb->insert_id;

Update:

$wpdb->update(
    $wpdb->prefix . 'my_plugin_log',
    [ 'message' => $new_message ],          // data
    [ 'id'      => $log_id ],               // where
    [ '%s' ],                               // data format
    [ '%d' ]                                // where format
);

Delete:

$wpdb->delete(
    $wpdb->prefix . 'my_plugin_log',
    [ 'item_id' => $item_id ],
    [ '%d' ]
);

Select — single row:

$row = $wpdb->get_row(
    $wpdb->prepare(
        "SELECT * FROM {$wpdb->prefix}my_plugin_log WHERE id = %d",
        $log_id
    )
); // returns stdClass or null

Select — multiple rows:

$rows = $wpdb->get_results(
    $wpdb->prepare(
        "SELECT * FROM {$wpdb->prefix}my_plugin_log WHERE item_id = %d ORDER BY created_at DESC LIMIT %d",
        $item_id,
        50
    )
); // returns array of stdClass

Select — single value:

$count = (int) $wpdb->get_var(
    $wpdb->prepare(
        "SELECT COUNT(*) FROM {$wpdb->prefix}my_plugin_log WHERE action = %s",
        'view'
    )
);

Raw query (DDL / no-result):

// phpcs:ignore WordPress.DB.DirectDatabaseQuery
$wpdb->query(
    $wpdb->prepare(
        "DELETE FROM {$wpdb->prefix}my_plugin_log WHERE created_at < %s",
        gmdate( 'Y-m-d H:i:s', strtotime( '-30 days' ) )
    )
);

4. Error handling

$result = $wpdb->insert( ... );
if ( false === $result ) {
    // $wpdb->last_error contains the MySQL error
    error_log( 'my-plugin DB error: ' . $wpdb->last_error );
    return new WP_Error( 'db_insert_error', $wpdb->last_error );
}

Enable query logging during development:

define( 'SAVEQUERIES', true );
// Then: print_r( $wpdb->queries )

5. Schema migrations (data migrations)

For migrating existing data (not just schema changes):

function my_plugin_migrate_to_1_2_0() {
    global $wpdb;

    // Example: move post meta to custom table
    $meta_rows = $wpdb->get_results(
        "SELECT post_id, meta_value FROM {$wpdb->postmeta} WHERE meta_key = '_my_plugin_data'"
    );

    if ( ! $meta_rows ) return;

    $table = $wpdb->prefix . 'my_plugin_log';
    foreach ( $meta_rows as $row ) {
        $wpdb->insert( $table, [
            'item_id' => $row->post_id,
            'message' => $row->meta_value,
            'action'  => 'migrated',
        ], [ '%d', '%s', '%s' ] );
    }

    // Remove the old meta after successful migration
    $wpdb->delete( $wpdb->postmeta, [ 'meta_key' => '_my_plugin_data' ], [ '%s' ] );
}

For large datasets, use batches (via wp-background-processing):

function my_plugin_migrate_batch( $offset = 0 ) {
    global $wpdb;
    $batch = $wpdb->get_results( $wpdb->prepare(
        "SELECT * FROM {$wpdb->postmeta} WHERE meta_key = '_old_key' LIMIT 100 OFFSET %d",
        $offset
    ) );
    // ... process batch ...
    if ( count( $batch ) === 100 ) {
        // More to process — schedule next batch
        as_enqueue_async_action( 'my_plugin_migrate_batch', [ 'offset' => $offset + 100 ], 'my-plugin' );
    } else {
        update_option( 'my_plugin_migration_complete', true );
    }
}

6. Table removal on uninstall

Use register_uninstall_hook (not deactivation_hook) for destructive cleanup:

// uninstall.php (registered via register_uninstall_hook(__FILE__, ...) or placed at plugin root)
if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) exit;

global $wpdb;

// Drop per-site tables on multisite
if ( is_multisite() ) {
    $sites = get_sites( [ 'number' => 0, 'fields' => 'ids' ] );
    foreach ( $sites as $site_id ) {
        switch_to_blog( $site_id );
        $wpdb->query( "DROP TABLE IF EXISTS {$wpdb->prefix}my_plugin_log" );
        delete_option( 'my_plugin_db_version' );
        restore_current_blog();
    }
} else {
    $wpdb->query( "DROP TABLE IF EXISTS {$wpdb->prefix}my_plugin_log" );
    delete_option( 'my_plugin_db_version' );
}

7. Seeding sample / preview data (dev only)

To populate custom tables with realistic data for local preview, write a standalone script run via WP-CLI's wp eval-file — never auto-loaded by the plugin. Keep it in a tools/ dir and document it in tools/README.md.

<?php
// tools/seed-dev-data.php — run: wp eval-file tools/seed-dev-data.php [--fresh]
if ( ! defined( 'WP_CLI' ) || ! WP_CLI ) { exit( "Run via: wp eval-file <this file>\n" ); }
global $wpdb;
$table = $wpdb->prefix . 'my_plugin_log';

// --fresh truncates first. TRUNCATE is destructive — gate it, and expect the
// agent permission classifier to block it unless tables are already empty.
if ( in_array( '--fresh', (array) ( $args ?? [] ), true ) ) {
    $wpdb->query( "TRUNCATE TABLE {$table}" ); // phpcs:ignore
}

foreach ( $rows as $row ) {
    $wpdb->insert( $table, $row ); // hardcoded columns only
}
WP_CLI::success( 'Seeded.' );

Conventions that keep seeders safe and re-runnable:

• Idempotent where it matters. A seeder that creates linked records (WP users, EDD payments) should skip rows already linked — e.g. if ( ! empty( $row->payment_id ) ) continue; — so reruns don't duplicate. A pure log-filler can be additive; say so in the script header and accept a count arg ((int) ( $args[0] ?? 0 ) ?: 20).
• Link to real WP objects, not fakes. Create real users with wp_insert_user() and reuse by email (get_user_by); mint EDD orders through the plugin's own purchase wrapper (e.g. an EDD integration class) rather than raw inserts, so the seeded data exercises the real code path. Write the resulting user_id / payment_id back onto the custom-table row.
• Generate via WP-CLI, verify via `wp db query`. Confirm row counts/links after seeding.
• Dev-only. Never ship tools/; never run against production. Use current_time('mysql') / gmdate() for timestamps, and seed extra/JSON columns with wp_json_encode().

Note on randomness: scripts run by wp eval-file may warn on large int math ($x * 2654435761 overflows to float) — keep PRNG seeds inside & 0x7fffffff.

Common Mistakes

Notes

• dbDelta() can ADD columns but cannot remove them. Removing columns requires ALTER TABLE DROP COLUMN in a manual migration step.
• Never use $wpdb->prepare() with %s for integers — always %d. Using %s on an integer is not a security issue but causes type coercion surprises.
• Column names in $wpdb->insert() / update() / delete() are NOT escaped — they must be hardcoded, never user-supplied.
• Cache expensive custom queries: $results = wp_cache_get( $cache_key, 'my_plugin' ); if ( false === $results ) { $results = $wpdb->get_results(...); wp_cache_set( $cache_key, $results, 'my_plugin', 300 ); }
• Always include bigint(20) unsigned NOT NULL AUTO_INCREMENT as the primary key — matches WP core table conventions.
• Use gmdate() not date() for DB timestamps; WP's current_time('mysql') returns local time — use it only when you need WP's configured timezone.

References

• references/dbdelta-rules.md — dbDelta() rules: strict SQL formatting requirements, column change limitations, and safe schema diff patterns
• references/migration-strategies.md — Versioned upgrade routines: db_version pattern, migration function registration, and idempotent migration checklist
• references/wpdb-patterns.md — $wpdb prepared statement patterns, insert/update/delete helpers, and custom-table query conventions