MySQL show relations MANY-MANY by condition - mysql

Question

There are two tables:
partner
id | name
--------------
1 | partner_1
2 | partner_2
3 | partner_3
4 | partner_4
contract
id | name | is_active
---------------------------
1 | contract_1 | 1
2 | contract_2 | 0
3 | contract_3 | 1
4 | contract_4 | 0
5 | contract_5 | 0
There is a third table that relates the previous two tables with many-to-many relationship
partner_contract
partner_id | contract_id
------------------------
1 | 1
1 | 2
2 | 3
2 | 2
2 | 4
3 | 5
Each partner can have several contracts, among which ONLY ONE can be active and some inactive .
Also the partner may not have contract at all.
I need a query that displays all the partners together with the active contract. If partner dont' have an active contract, display NULL.
partner_id | partner_name | contract_name
-----------------------------------------
1 | partner_1 | contract_1
2 | partner_2 | contract_3
3 | partner_3 | NULL
4 | partner_4 | NULL
I found a solution, but it seems to me that it is not perfect .
SELECT
p.id AS partner_id,
p.name AS partner_name,
active_contract.name AS contract_name
FROM partner p
LEFT JOIN (
SELECT *
FROM contract c
LEFT JOIN partner_contract pc on pc.contract_id = c.id
WHERE c.is_active = 1
) active_contract
ON active_contract.partner_id = p.id
Is there a more elegant solution?

Answer 1

Ray's (deleted) query is close to the right solution. The condition on the contract should go in the on clause, not the where clause:
SELECT p.id AS partner_id, p.name AS partner_name, c.name AS contract_name
FROM partner p LEFT JOIN
partner_contract pc
ON p.id = pc.partner_id LEFT JOIN
contract c
ON pc.contract_id = c.id AND c.is_active = 1;
EDIT:
Okay, the above is wrong. This can be fixed with a group by:
SELECT p.id AS partner_id, p.name AS partner_name, MAX(c.name) AS contract_name
FROM partner p LEFT JOIN
partner_contract pc
ON p.id = pc.partner_id LEFT JOIN
contract c
ON pc.contract_id = c.id AND c.is_active = 1
GROUP BY p.id, p.name;
The more elegant solution (in my opinion):
SELECT p.*,
(select name
from partner_contract pc join
contract c
on pc.contract_id = c.id AND c.is_active = 1
where p.id = pc.partner_id
) as contract_name
FROM partner p;
SQL Fiddle
This can take advantage of indexes and does not require aggregation.