Calculate the amount payable to each user - mysql

Question

I want to calculate the amount payable to each user.
This may be in the negative. Briefly:
MustPay = AmountTaken - AmountPaid
I could not write the sql query.
SELECT users.Name, users.Surname,
SUM(takenfrom.AmountTaken) - SUM(paid.AmountPaid) AS MustPay
FROM users
LEFT JOIN takenfrom ON takenfrom.UserId = users.UserId
LEFT JOIN paid ON paid.UserId = users.UserId
GROUP BY users.UserId
Tables:
FIRST TABLE USERS
| UserId | Name | Surname |
| 1 | foo | boo |
| 2 | f | b |
SECOND TABLE TAKENFROM
| TakenFromId | UserId | AmountTaken|
| 1 | 1 | 100 |
| 2 | 2 | 200 |
THIRD TABLE PAID
| PaidId | UserId | AmountPaid|
| 1 | 2 | 50 |
| 2 | 2 | 50 |
RESULT TABLE
| Name | Surname| MustPay |
| foo | boo | 100 |
| f | b | 100 |

Answer 1

You don't need a LEFT JOIN on TakenFrom as every user is going to have an amount billed to them, whether or not they've paid is what Paid is for. You only need a LEFT JOIN on Paid, as user may have paid already or they may have not.
Since not every user is going to have an AmountPaid, so you need to use an IFNULL() to check that. SUM() returns NULL if it's given a NULL.
Also, unless there are multiple rows for each user in takenfrom, then you don't need a SUM() for AmountTaken.
SELECT users.Name, users.Surname,
SUM(takenfrom.AmountTaken) - IFNULL(SUM(paid.AmountPaid), 0) AS MustPay
FROM users
JOIN takenfrom ON takenfrom.UserId = users.UserId
LEFT JOIN paid ON paid.UserId = users.UserId
GROUP BY users.UserId
DEMO: http://sqlfiddle.com/#!9/e69b3b/1
UPDATE: If both paid and takenfrom have multiple rows (for a UserId), then you'll get duplicate rows from the JOINs. To fix this, you can use subqueries instead of JOIN:
SELECT Name, Surname,IFNULL((
SELECT SUM(AmountTaken) FROM takenfrom WHERE UserID = users.UserID
), 0) - IFNULL((
SELECT SUM(AmountPaid) FROM paid WHERE UserID = users.UserID
), 0) AS MustPay
FROM users
DEMO: http://sqlfiddle.com/#!9/3960b5/26