Finding rows that exist in one table but not another

There are two ways to find rows that exist in one table but not another.

The first method is to use a subquery:

SELECT id FROM table1 WHERE id NOT IN (SELECT id FROM table2);

This approach has the advantage of being more obvious to the casual reader, at the cost of portability (you need MySQL version 4.1 to use subqueries).

Alternately, you can use a left join:

SELECT table1.id FROM table1 LEFT JOIN table2 ON (table1.id = table2.id) WHERE table2.id IS NULL;

This approach may execute faster than a subquery, and works in all versions of MySQL. It may lose clarity for those readers not familiar with table joins and null values.