MYSQL - Select only if row in LEFT JOIN is not present

Question

I have 2 simple mysql tables. The first 1 called mail and has 2 rows:

sender | receiver
Marley | Bob 
Saget  | Bob 

The second one called b

Answer 1

Try this:

SELECT sender
FROM mail m
WHERE NOT EXISTS (SELECT 1 FROM block 
                  WHERE blocker = m.receiver 
                  AND blocked = m.sender)