Question

I have 2 tables.

How do I know IDs that are in one table and not on the other table? How would I accomplish that?

And then I want to delete all such IDs.

Answer 1

This is pretty simple:

delete from t1
using table1 as t1
left outer join table2 as t2
on t1.id = t2.id
where t2.id is null

It’s worth noting that joins are faster than subqueries.