Question

I am trying to unify a pair of queries with a LEFT JOIN, so that I can perform an ORDER BY on a desired column.

Table A has an exclusive one-to-many relationship with Table B, and table B has an exclusive one-to-many relationship with Table C.

I want to get the maximum value of a column in Table C, for each row in Table A.

what I used to have was:

$tableA = SELECT * FROM tableA;
for each row in $tableA {
  $maxValue = SELECT MAX(value) FROM tableC WHERE tableB_id IN (SELECT tableB_id FROM tableB WHERE tableA_id={$row['tableA_id']}) GROUP BY tableB_id;
}

and now I’m thinking along the lines of:

SELECT * FROM tableA LEFT JOIN (SELECT tableA_id, MAX(max_c_value) FROM (SELECT tableB_id, MAX(value) max_c_value FROM tableC GROUP BY tableB_id) t GROUP BY tableA_id) USING(tableA_id)

but I know that’s gibberish. I am not sure where to go from here.
I’m finding it hard to explain the problem, sorry.

Answer 1

SELECT A.ID, MAX(C.MyField) as CField
FROM A
    LEFT OUTER JOIN B
        ON A.ID = B.A_ID
            LEFT OUTER JOIN C
                ON B.ID = C.B_ID
GROUP BY A.ID

You will get null value for CField if there is no rows corresponding.