Select data that exists in A but not in B

Question

Imagine I have this tables: .

What I need is to get the Data that exists in A but not in B, in this case my SELECT will have to return "2".

I've done it before, but right know I can't remember how. I suppose it was something like this:

SELECT a.*
FROM A as a
LEFT JOIN B AS b ON b.column = a.column

But it's not working. Can someone help me, please?

Thanks in advance.

Answer 1

You're just missing a filter:

SELECT a.*
FROM A as a
LEFT JOIN B AS b ON b.column = a.column
WHERE B.column IS NULL

Answer 2

If B could have multiple rows that match A, then this query would be more appropriate:

SELECT a.*
FROM A as a
WHERE NOT EXISTS(SELECT NULL FROM B WHERE b.column = a.column)

Answer 3

the following should work for most database types

select *
from A
where A.column not in (select column from B)