Question

For each user, I want to produce an aggregate result. However, I keep running into the problem of the subquery returning more than 1 result.

SELECT **[user]**, 
   ((SELECT SUM(amount) 
          FROM transactions 
          WHERE [user] = **[user]** 
          AND [type] = 'credit' ) - 
            (SELECT SUM(amount) 
            FROM transactions 
            WHERE [user] = **[user]** 
            AND [type] = 'debit' )) 
     FROM transactions

How do I get the user from the select at the start to then operate as the variable user in the subqueries?

Answer 1

use CASE to simplify the summing:

SELECT user,SUM(CASE type WHEN 'credit' THEN amount WHEN 'debit' THEN -amount END)
from transactions
GROUP BY user

If credit and debit are the only valid types, then it can be simplified further as:

CASE type WHEN 'credit' THEN amount ELSE -amount END

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The answers using subqueries for the summing activity, whilst still selecting user from the transactions table will perform the summation multiple times – once for every row in the transactions table – so if one user has 5 transactions, they’re going to compute the sums 5 times (and show 5 result rows), which I assume you do not want.