MySQL snippet - Selecting from table a items that aren't in table b.

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.

Recent posts View all

Rails

Active Storage - resizing images on upload

Our fix for app timeouts due to upload of large images when Active Storage was used.

iOS

iOS simulator battery level

I have always enjoyed this fun feature