Do aggregate MySQL functions always return a single row?

Question

I\'m sorry if this is really basic, but:

I feel at some point I didn\'t have this issue, and now I am, so either I was doing something totally different before or my

Answer 1

You need to use GROUP BY as such to get your desired result:

SELECT
order_id,
part_id,
SUM(cost) AS total
FROM orders 
WHERE order_date BETWEEN xxx AND yyy
GROUP BY order_id, part_id

This will group your results. Note that since I assume that order_id and part_id is a compound PK, SUM(cost) in the above will probably be = cost (since you a grouping by a combination of two fields which is guarantied to be unique. The correlated subquery below will overcome this limitation).

Any non-aggregate rows fetched needs to be specified in the GROUP BY row.

For more information, you can read a tutorial about GROUP BY here:

MySQL Tutorial - Group By

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EDIT: If you want to use a column as both aggregate and non-aggregate, or if you need to desegregate your groups, you will need to use a subquery as such:

SELECT
or1.order_id,
or1.cost,
or1.part_id,
(
  SELECT SUM(cost)
  FROM orders or2
  WHERE or1.order_id = or2.order_id
  GROUP BY or2.order_id
) AS total
FROM orders or1
WHERE or1.order_date BETWEEN xxx AND yyy