Question

So I store all transactions in a transaction table with the following struture:

+----------------+---------------+------+-----+---------+----------------+
| Field          | Type          | Null | Key | Default | Extra          |
+----------------+---------------+------+-----+---------+----------------+
| debit_amount   | decimal(10,2) | YES  |     | 0.00    |                | 
| credit_amount  | decimal(10,2) | YES  |     | 0.00    |                | 
| flag           | int(11)       | YES  |     | NULL    |                | 
| date           | datetime      | YES  |     | NULL    |                | 
| id             | int(11)       | NO   | PRI | NULL    | auto_increment | 
+----------------+---------------+------+-----+---------+----------------+

Then I store the total amount of credits the user has in the “credits” row in the user table.

I am trying to figure out whether there is a mismatch in the total(debit amount + credit amount) for each user as stored in the transactions table to the number of credits stored in the user table.

basically for each user

 transactions.debit_amount + transactions.credit amount MUST EQUAL user.credits

but the does not equal operator in the mysql query does not work (especially when the transactions.total is null i.e. there is no row in the transactions table for that user):

SELECT s.id AS uid, s.total, s.credits
FROM (
  SELECT (sum(t.credit_amount) + sum(t.debit_amount)) AS total, t.userid, u.credits, u.id
  FROM transactions AS t
  RIGHT JOIN users AS u ON t.userid = u.id 
  GROUP BY u.id
) AS s
WHERE s.total != s.credits

Answer 1

Try:

select u.id, u.credits, t.total
from  users u
left join (
    select userid, sum(coalesce(credit_amount,0)) + sum(coalesce(debit_amount, 0)) as total
    from transactions
    group by userid
) t on u.id = t.userid
where coalesce(t.total, 0) <> coalesce(u.credits, 0)