Delete records not present in join

Question

Imagine two tables (A and B):

A        B
1        2
2        3 
6        5 
4        7
9        11
         13
         23
         9 

Now I want to delete records from A that are not present in B, e.g deleting 1, 6 and 4 from A.

My initial idea is that you could 'negate' the results of a join.

Answer 1

DELETE FROM A WHERE NOT EXISTS (SELECT * FROM B WHERE A.id = B.id)

I've assumed that those columns are named id.

Answer 2

An alternative to the NOT EXISTS answer:

DELETE FROM A WHERE id NOT IN (SELECT id FROM b);

Again, assuming the column is named id.