I have a two tables - items and its states. Each item has only one state. I need to get list of states with counted items of that state, but only from certain ower. When I remove WHERE part I get list of all states, but with it I get only some of states, because there are not at all same states used in items with owner no.7. How to get all of states even if it's amount is zero?
Only solution I have is use subquery instead of COUNT(i.id), but that would be much more slower and I am afraid of performance.
My tables:
ID STATE NAME
== ==========
1 New
2 Used
3 Archived
4 Unknown
ID STATE ITEM NAME OWNER
== ===== ========= =====
1 1 ABC 7
2 2 DEF 6
3 3 GHI 7
4 1 JKL 7
My query:
SELECT
s.id,
s.name,
COUNT(i.id) AS count
FROM
b_items_states s
LEFT JOIN
b_items i ON i.state = s.id
WHERE
i.owner = 7
GROUP BY
s.id
My result:
ID NAME COUNT
== ==== =====
1 New 2
3 Archived 1
My expected result:
ID NAME COUNT
== ==== =====
1 New 2
2 Used 0
3 Archived 1
4 Unknown 0
Move the filtering condition inside the JOIN, as in:
SELECT
s.id,
s.name,
COUNT(i.id) AS count
FROM
b_items_states s
LEFT JOIN
b_items i ON i.state = s.id AND i.owner = 7
GROUP BY
s.id
If you place the filtering condition in a WHERE statement, you are implicitly converting the outer join into an inner join, without even noticing.