无法为Wordpress插件创建MYSQL表

GRANT ALL PRIVILEGES ON database_name.* TO 'username'@'localhost';
FLUSH PRIVILEGES;

SELECT table_schema AS `Database`, SUM(data_length + index_length) / 1024 / 1024 AS `Size (MB)` FROM information_schema.TABLES GROUP BY table_schema;

<?php
/*
Plugin Name: Example Plugin
*/

function create_example_table() {
    global $wpdb;
    $table_name = $wpdb->prefix . 'example_table';

    if ($wpdb->get_var("SHOW TABLES LIKE '$table_name'") != $table_name) {
        $charset_collate = $wpdb->get_charset_collate();

        $sql = "CREATE TABLE $table_name (
            id mediumint(9) NOT NULL AUTO_INCREMENT,
            name varchar(50) NOT NULL,
            PRIMARY KEY (id)
        ) $charset_collate;";

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