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.