Question

Consider the following table:

mysql> select * from phone_numbers; +-------------+------+-----------+ | number      | type | person_id | +-------------+------+-----------+ | 17182225465 | home |         1 | | 19172225465 | cell |         1 | | 12129876543 | home |         2 | | 13049876543 | cell |         2 | | 15064223454 | home |         3 | | 15064223454 | cell |         3 | | 18724356798 | home |         4 | | 19174335465 | cell |         5 | +-------------+------+-----------+ 

I’m trying to find those people who have home phones but not cells.

This query works:

mysql> select h.*     -> from phone_numbers h     -> left join phone_numbers c     -> on h.person_id = c.person_id     -> and c.type = 'cell'     -> where h.type = 'home'     -> and c.number is null; +-------------+------+-----------+ | number      | type | person_id | +-------------+------+-----------+ | 18724356798 | home |         4 | +-------------+------+-----------+ 

but this one doesn’t:

mysql> select h.*     -> from phone_numbers h     -> left join phone_numbers c     -> on h.person_id = c.person_id     -> and h.type = 'home'     -> and c.type = 'cell'     -> where c.number is null; +-------------+------+-----------+ | number      | type | person_id | +-------------+------+-----------+ | 19172225465 | cell |         1 | | 13049876543 | cell |         2 | | 15064223454 | cell |         3 | | 18724356798 | home |         4 | | 19174335465 | cell |         5 | +-------------+------+-----------+ 

The only difference between the two is the location of the h.type = 'home' condition – in the first it’s in the where clause and in the second it’s part of the on clause.

Why doesn’t the second query return the same result as the first?

Answer 1

In the second SQL, the condition h.type = ‘home’ is part of the outer join conditions, and is not a filter on the results. For all records where h.type=’cell’, the condition h.type = ‘home’ is FALSE and so no ‘matching’ c row is found – so c.number is null, which is your only filtering (WHERE) condition.

In pseudo-code your 2nd SQL works like this:

for each row in phone_numbers h /* Note this is ALL home AND cell phones */    select c.number from phone_numbers c    where h.person_id = c.person_id    and h.type = 'home'    and c.type = 'cell';    if c.number is null (i.e. no row found)      display h.*    end if end loop;