Question

I have two entities with an 1-N relation, like this :

table_a
-------
id
name

table_b
------
id
table_a_id
name
status
created_at  

I’m looking for a way in MySQL and especially with Doctrine ORM to query table_a with a “where” clause on table_b that affect only the last associated table_b record.

Supposing I have the following records :

table_a
----------------------------
id    | name
----------------------------
1     | john
2     | mary
3     | chuck

table_b
--------------------------------------------------
id    | table_a_id | name    | status | created_at
--------------------------------------------------
1     | 1          | blue    | 1      | 2000-01-01
2     | 1          | red     | 1      | 2012-12-31
3     | 2          | yellow  | 1      | 2000-01-01
4     | 2          | green   | 0      | 2012-12-31

So I want to tell MySQL/Doctrine :
GIVE ME the table_a records
WHICH HAVE table_b records
AND status = 1 ON the last related elements (according to the created_at field)

This should only return :

table_a
----------------------------
id    | name
----------------------------
1     | john

Answer 1

According to the book SQL Antipatterns, this type of join with the proper indexes can often perform better than a subquery. So, try this method out too:

SELECT a.*
FROM table_a a
JOIN table_b b1
  ON b1.table_a_id = a.id
  AND b1.status = 1
LEFT JOIN table_b b2
  ON b2.table_a_id = a.id
  AND b2.created_at > b1.created_at
WHERE b2.table_a_id IS NULL

If there could be two rows from table_b with status 1 that have the same table_a_id and created_at date, then you will need DISTINCT to avoid duplicates:

SELECT DISTINCT a.*
FROM table_a a
JOIN table_b b1
  ON b1.table_a_id = a.id
  AND b1.status = 1
LEFT JOIN table_b b2
  ON b2.table_a_id = a.id
  AND b2.created_at > b1.created_at
WHERE b2.table_a_id IS NULL