Question

I would like some help with the following join.
I have one table (with about 20 million rows) that consists of:

MemberId (Primary Key) | Id (Primary Key) | TransactionDate | Balance

I would like to get the latest Balance for all the customers in one query.
I know I could do something like this (I just wrote it from my memory). But this way is terribly slow.

SELECT * 
FROM money 
WHERE money.Id = (SELECT MAX(Id) 
                  FROM money AS m 
                  WHERE m.MemberId = money.MemberId)

Are there any other (faster/smarter) options?

Answer 1

In all optimization tutorials and screencasts that I’ve endured through, joins are always favoured over subqueries. When using a sub-query the sub-query is executed for each comparison, where as with a join only once.

SELECT * 
FROM money m
INNER JOIN (
    SELECT memberId, MAX(id) AS maxid
    FROM money
    GROUP BY memberId
) mmax ON mmax.maxid = m.id AND mmax.memberId = m.memberId