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Finding rows that exist in one table but not another

How do you find all the rows in one table that do not exist in another table?

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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.

This was first published in July 2006

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