global $myplugin_db_version;
$myplugin_db_version = "1.0";

function myplugin_install() {
    global $wpdb;
    global $myplugin_db_version;

    $table_name = $wpdb->prefix . "myplugin";

    $installed_ver = get_option( "myplugin_db_version" );

    if( $installed_ver != $myplugin_db_version ) {

        $sql = "CREATE TABLE $table_name (
                id mediumint(9) NOT NULL AUTO_INCREMENT,
                time datetime DEFAULT \'0000-00-00 00:00:00\' NOT NULL,
                name tinytext NOT NULL,
                text text NOT NULL,
                url VARCHAR(100) DEFAULT \'\' NOT NULL,
                UNIQUE KEY id (id));
        ";         

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

        update_option( "myplugin_db_version", $myplugin_db_version );

    }
}

function myplugin_update_db_check() {
    global $myplugin_db_version;
    if ( get_site_option( \'myplugin_db_version\' ) != $myplugin_db_version) {
        myplugin_install();
    }
}
add_action(\'plugins_loaded\', \'myplugin_update_db_check\');