Finding 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.