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)

Answer 2

The left join will produce null rows for the mismatches.
It's those null rows that you need to filter on.

SELECT * FROM mail  
LEFT JOIN block ON (block.blocker = 'Bob') 
WHERE block.blocker IS NULL

It's kind of strangle to be joining on a fixed value however, a more common join (given your tables) would be:

SELECT * FROM mail  
LEFT JOIN block ON (block.blocker = mail.receiver
                and block.blocked = mail.sender)<<-- these should match
WHERE block.blocker IS NULL                     <<-- select only mismatches
AND mail.receiver like 'bob';