Question

Assuming the following MySQL table structure, why do the two following queries produce different results?

games(id) (464 records)
members(id) (1 record, id=351)
gameslists(id,memberid,gameid) -- (2 records, (1,351,1) and (2,351,2))

This produces null

SELECT games.* 
FROM games 
INNER JOIN gameslists ON gameslists.gameid = games.id 
WHERE gameslists.memberid <> 351 AND gameslists.id is NULL

This produces 462 records, which is what I expect.

SELECT games.* 
FROM games
LEFT JOIN gameslists ON gameslists.gameid = games.id AND gameslists.memberid <> 351
WHERE gameslists.id is NULL 

Answer 1

The expression (gameslists.id is NULL) can never be true in the INNER JOIN query (assuming id is the primary key). That’s why the first result set contains no rows.

On the other hand, whenever the ON clause of the LEFT JOIN does not match, the gameslists fields will be NULL for that particular row. Therefore your second query will return all the games that do not appear in gameslists, unless memberid is 351.