Question

MySql 5.5.

I have a table that represents a work assignment:

   empId   jobNo   workDate   hours
     4      441    10/1/2012    10
     4      441     9/1/2012    22
     4      441     8/1/2012     6

And one that represents salary:

   empId   effDate     rate
     4     10/1/2012    6.50
     4      9/1/2012    5.85
     4      6/1/2012    4.00

The salary applies to all work performed on or after the effective date. So the rate in jun, jul, and aug is 4.00; sep is 5.85, and oct is 6.50.

If I naively query for October’s work:

   SELECT Work.empId, Work.jobNo, Work.workDate, Work.hours, Salary.effDate, Salary.rate
   FROM Work
   JOIN Salary ON Work.empId = Salary.empId
   WHERE Work.workDate <= '2012-10-01'
      AND Salary.effDate <= Work.workDate
   ORDER BY Work.jobNo ASC, Work.workDate DESC;

I do not get what I want. I get something like

   4   441   10/1/2012   10   10/1/2012   6.50
   4   441   10/1/2012   10    9/1/2012   5.85
   4   441   10/1/2012   10    6/1/2012   4.00
   4   441    9/1/2012   22    9/1/2012   5.85
   4   441    9/1/2012   22    6/1/2012   4.00
   4   441    8/1/2012    6    6/1/2012   4.00

When I want

   4   441   10/1/2012   10   10/1/2012   6.50
   4   441    9/1/2012   22    9/1/2012   5.85
   4   441    8/1/2012    6    6/1/2012   4.00

I can’t quite wrap my head around how to create the query I want.
The real situation has multiple employees, multiple jobs, obviously.

Thanks for your help.

Answer 1

Here is your actual issue: you want to be able to detect, for each record in Work, what is the corresponding effective rate, according to the work date x salary effective date. When you simply do Salary.effDate <= WORK.workDate you get ALL rates before the work date. But you only want the most recent one.

This is a slightly complicated variant of the greatest-n-per-group problem. There are many ways of doing this, here is one:

SELECT sel.*, Salary.Rate
FROM
(
     SELECT Work.empId, Work.jobNo, Work.workDate, 
           Work.hours, Max(Salary.effDate) effDate
    FROM WORK
    JOIN Salary ON WORK.empId = Salary.empId
    WHERE WORK.workDate <= '2012-10-01'
       AND Salary.effDate <= WORK.workDate
    GROUP BY WORK.empId, WORK.jobNo, WORK.workDate, WORK.hours
    ORDER BY WORK.jobNo ASC, WORK.workDate DESC
) sel 
INNER JOIN Salary ON sel.empId = Salary.empId 
                  AND sel.EffDate = Salary.EffDate

First of all, the inner query detects the most recent salary effective date for each work record. Then, we join that with the Salary again to the rate.

See the working SQLFiddle.