Sums are being multiplied in query - mysql

Question

I am working on 2 problems for homework and after many hours I have just about solved them both, the last issue I have is that both of my queries are coming back with doubled numerical values instead of single.
Here is what I have:
SELECT SUM(P.AMT_PAID) AS TOTAL_PAID, C.CITATION_ID, C.DATE_ISSUED, SUM(V.FINE_CHARGED) AS TOTAL_CHARGED
FROM PAYMENT P, CITATION C, VIOLATION_CITATION V
WHERE V.CITATION_ID = C.CITATION_ID
AND C.CITATION_ID = P.CITATION_ID
GROUP BY C.CITATION_ID;
and my other one:
SELECT C.CITATION_ID, C.DATE_ISSUED, SUM(V.FINE_CHARGED) AS TOTAL_CHARGED, SUM(P.AMT_PAID) AS TOTAL_PAID, SUM(V.FINE_CHARGED) - SUM(P.AMT_PAID) AS TOTAL_OWED
FROM (CITATION C)
LEFT JOIN VIOLATION_CITATION V
ON V.CITATION_ID = C.CITATION_ID
LEFT JOIN PAYMENT P
ON P.CITATION_ID = C.CITATION_ID
GROUP BY C.CITATION_ID
ORDER BY TOTAL_OWED DESC;
I am sure there is just something that I am overlooking. If someone else could kindly tell me where I went awry it would be a great help.

Answer 1

Select Sum(P.Amt_Paid) As Total_Paid, C.Citation_Id
, C.Date_Issued, Sum(V.Fine_Charged) As Total_Charged
From Payment P
Join Citation C
On C.Citation_Id = P.Citation_Id
Join Violation_Citation V
On V.Citation_Id = C.Citation_Id
Group By C.Citation_Id
First, you should use the JOIN syntax instead of using the comma-delimited list of tables. It makes it easier to read, more standardized and will help prevent problems by overlooking a filtering clause.
Second, the most likely reason for having a sum that is too large is due to the join to the VIOLATION_CITATION table. If you remove the Group By and columns with aggregate functions, you will likely see that P.AMT_PAID is repeated for each instance of VIOLATION_CITATION. Perhaps, the following will solve the problem:
Select Coalesce(PaidByCitation.TotalAmtPaid,0) As Total_Paid
, C.Citation_Id, C.Date_Issued
, Coalesce(ViolationByCitation.TotalCharged,0) As Total_Charged
, Coalesce(ViolationByCitation.TotalCharged,0)
- Coalesce(PaidByCitation.TotalAmtPaid,0) As Total_Owed
From Citation As C
Left Join (
Select P.Citation_Id, Sum( P.Amt_Paid ) As TotalAmtPaid
From Payment As P
Group By P.Citation_Id
) As PaidByCitation
On PaidByCitation.Citation_Id = C.Citation_Id
Left Join (
Select V.Citation_Id, Sum( V.Find_Charged ) As TotalCharged
From Violation_Citation As V
Group By V.Citation_Id
) As ViolationByCitation
On ViolationByCitation.Citation_Id = C.Citation_Id
The use of Coalesce is to ensure that if the left join returns no rows for a given Citation_ID value, that we replace the Null with zero.