Table Prefix

Set once – applied automatically to every table name in every query.

What is a table prefix?

A table prefix is a short string that gets prepended to every table name in your queries. It is common in shared hosting environments or CMS systems like WordPress (wp_) where multiple applications share one database.

Without a prefix you always write the full table name. With a prefix set, you only write the base name – PDOdb adds the prefix automatically: 

// Without prefix
$users = $db->get('wp_users');
// → SELECT * FROM wp_users

// With prefix 'wp_' set
$users = $db->get('users');
// → SELECT * FROM wp_users

Setting the prefix

Pass the prefix in the constructor:

$db = new PDOdb([
    'host'     => 'localhost',
    'username' => 'user',
    'password' => 'secret',
    'db'       => 'my_database',
    'prefix'   => 'wp_',
]);

Or set / change it at any time after initialization:

$db->setPrefix('app_');

// Read the current prefix
$prefix = $db->getPrefix(); // → 'app_'

Where the prefix is applied

The prefix is automatically added in all of the following:

$db->setPrefix('tst_');

// get(), getOne(), getValue()
$db->get('users');
// → SELECT * FROM tst_users

// insert(), update(), delete(), replace()
$db->insert('orders', ['user_id' => 1, 'total' => 49.99]);
// → INSERT INTO tst_orders (user_id, total) VALUES (?, ?)

// join()
$db->join('orders o', 'o.user_id = users.id', 'LEFT')->get('users');
// → SELECT * FROM tst_users LEFT JOIN tst_orders o ON o.user_id = users.id

// paginate()
$db->paginate('users', 1);
// → SELECT SQL_CALC_FOUND_ROWS * FROM tst_users LIMIT 0, 20

// tableExists()
$db->tableExists('users');
// → checks for tst_users in information_schema

Prefix and subqueries

Subquery objects created via subQuery() do not inherit the prefix automatically. Use the full table name inside a subquery: 

$db->setPrefix('tst_');

$sub = $db->subQuery();

// ❌ Wrong – prefix is not applied inside subQuery()
$sub->where('status', 'active')->get('orders', null, 'user_id');

// ✅ Correct – use the full table name
$sub->where('status', 'active')->get('tst_orders', null, 'user_id');

$rows = $db->where('id', $sub, 'IN')->get('users');
// → SELECT * FROM tst_users WHERE id IN (SELECT user_id FROM tst_orders WHERE status = ?)
This is a known limitation. Subquery instances are created without a database connection and therefore cannot inherit the prefix from the parent instance.

Prefix in rawQuery()

rawQuery() automatically adds the prefix to table names that appear after FROM, INTO, UPDATE, JOIN and DESCRIBE. System tables like information_schema are never prefixed. 

$db->setPrefix('tst_');

$db->rawQuery("SELECT * FROM users WHERE active = ?", [1]);
// → SELECT * FROM tst_users WHERE active = 1

$db->rawQuery("SELECT * FROM information_schema.COLUMNS WHERE ...");
// → SELECT * FROM information_schema.COLUMNS WHERE ...
//   (system tables are never prefixed)

See also