What are some common pitfalls when using MySQL queries in PHP with multiple tables and how can they be avoided?

One common pitfall when using MySQL queries in PHP with multiple tables is not properly specifying the table names in the query, which can lead to ambiguous column names and incorrect results. To avoid this, always use table aliases and prefix column names with the table alias in the SELECT statement.

// Incorrect query without table aliases
$query = "SELECT id, name FROM users, orders WHERE users.id = orders.user_id";

// Correct query with table aliases
$query = "SELECT u.id, u.name FROM users u, orders o WHERE u.id = o.user_id";