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I have 3 tables (attendance, allowances and deductions) with some records in attendance.Wage, allowances.Amount, deductions.Amount columns. And I want to "SUM" values in these columns with selected date.

Summed up values must be

  • attendance.Wage:100
  • allowances.Amount:150
  • deductions.Amount:120

but with my query values are seeing very different.

SELECT Name, SUM(attendance.Wage), SUM(allowances.Amount), SUM(deductions.Amount) FROM employees
INNER JOIN attendance USING (EmployeeID)
INNER JOIN allowances USING (EmployeeID)
INNER JOIN deductions USING (EmployeeID)
WHERE MONTH(attendance.Date) = 6 AND YEAR(attendance.Date) = 2020 
AND
MONTH(allowances.Date) = 6 AND YEAR(allowances.Date) = 2020 
AND
MONTH(deductions.Date) = 6 AND YEAR(deductions.Date) = 2020 
GROUP BY employees.EmployeeID;

Output of the query:

  • attendance.Wage:400
  • allowances.Amount:900
  • deductions.Amount:720

Why the values are multiplying or increasing? How can I fix that?

GMB
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Ulrich
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  • Please in code questions give a [mre]--cut & paste & runnable code, including smallest representative example input as code; desired & actual output (including verbatim error messages); tags & versions; clear specification & explanation. Give the least code you can that is code that you show is OK extended by code that you show is not OK. (Debugging fundamental.) For SQL that includes DBMS & DDL (including constraints & indexes) & input as code formatted as a table. [ask] Pause work on the overall goal, chop code to the 1st expression not giving what you expect & say what you expect & why. – philipxy Jun 16 '20 at 23:00

2 Answers2

1

Because you are getting multiple rows from each table and the join is multiplying them.

Without additional information, I would recommend correlated subqueries:

SELECT e.Name,
       (SELECT SUM(a.Wage)
        FROM attendance a
        WHERE a.EmployeeID = e.EmployeeID AND
              a.date >= '2020-06-01' AND a.date < '2020-07-01'
       ),
       (SELECT SUM(a.Amount)
        FROM allowances a
        WHERE a.EmployeeID = e.EmployeeID AND
              a.date >= '2020-06-01' AND a.date < '2020-07-01'
       ),
       (SELECT SUM(d.Amount)
        FROM deduction d
        WHERE d.EmployeeID = e.EmployeeID AND
              d.date >= '2020-06-01' AND d.date < '2020-07-01'
       )
FROM employees e;

With an index on (EmployeeId, date, amount/wage) in each of the three tables, this should also have better performance than alternatives using explicit aggregations and joins.

Gordon Linoff
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  • Thanks. Now I realized. But I can't understand why this have better performance than alternatives using explicit aggregations and joins? (I wan't to learn this too) – Ulrich Jun 16 '20 at 22:32
  • @user13758994 . . . Replacing the outer aggregation with look ups in an index is why the performance is probably going to be better. The only exception would be if you had a really large `employees` table and most of them had no records in any of the tables in those months. – Gordon Linoff Jun 16 '20 at 23:23
0

You would need to push the aggregation down in subqueries, otherwise the sums count each value multiple times.

SELECT e.Name, ad.total_attendance, aw.total_allowances, dd.total_deductions
FROM employees e
INNER JOIN (
    SELECT EmployeeID, SUM(wage) total_attendance
    FROM attendance 
    WHERE date >= '2020-06-01' and date < '2020-07-01'
    GROUP BY EmployeeID
) ad USING (EmployeeID)
INNER JOIN (
    SELECT EmployeeID, SUM(amount) total_allowances
    FROM allowances 
    WHERE date >= '2020-06-01' and date < '2020-07-01'
    GROUP BY EmployeeID
) aw USING (EmployeeID)
INNER JOIN (
    SELECT EmployeeID, SUM(amount) total_deductions
    FROM deductions 
    WHERE date >= '2020-06-01' and date < '2020-07-01'
    GROUP BY EmployeeID
) dd USING (EmployeeID)
GMB
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