A喜欢B，但是B不喜欢A？

Question

If my table, called Table, looks like this:

A B
1 2
3 2
2 1
etc...

This would imply 1 likes 2, 3 likes 2, and 2 likes 1...assuming its a lot bigger than this, how do I write an SQL query where I report where A likes B, but B does not like A?

So for this case, an example output should be:

3 2

Because 3 likes 2 from the relation, but 2 does not like 3.

Answer 1

SELECT A, B
  FROM likes x
 WHERE NOT EXISTS (SELECT * FROM likes
                    WHERE x.A = B AND x.B = A)

Answer 2

I think you just need to use a left join (Disclaimer: I use SQL Server not postgresql)

select l.A, l.B
from likes l
  left join likes nl on l.B=nl.A and l.A=nl.B
where nl.A is null