MySQL snippet - Selecting from table a items that aren't in table b
How I found items that were present in one table but not another
Today I needed to select something from a table on the condition that the ID of the row in the table did not appear in another table.
Initially I thought this was going to be slightly difficult to do, or at least a bit convoluted but it turned out to be really simple.
SELECT table_a.id FROM table_a LEFT JOIN table_b ON table_a.id = table_b.aid WHERE table_b.aid IS NULL;
Since you are using a LEFT JOIN table_b.aid can return NULL values, and these are the ones you are looking for.