SQL query by date - mysql

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.

Answer 2

You're using what's called a NATURAL JOIN. Try changing the word "JOIN" to "LEFT JOIN" which should group the results on the left, giving you the desired results.

Answer 3

Assuming the salary table has a primary or alternate key (unique index) consisting of the columns empId and effDate, I'd do something like this:
select w.empID as EMPLOYEE_ID ,
w.jobNo as JOB_NUMBER ,
w.workDate as DATE_WORKED ,
w.hours as HOURS_WORKED ,
rate.HourlyWage as HOURLY_WAGE ,
w.hours * rate.HourlyWage as WAGES_CHARGED ,
rate.effDateFrom as HOURLY_WAGE_EFFECTIVE_DATE
from work w
join ( select sfrom.EmpId as EmpID ,
sfrom.rate as HourlyWage ,
sfrom.EffDate as effDateFrom ,
( select min(Effdate)
from salary t
where t.empId = sfrom.EmpId
and t.effDate > sfrom.EffDate
) as effDateThru
from salary sfrom
) rate on rate.empID = w.empID
and rate.EffDateFrom <= w.workDate
and ( rate.effDateThru is null -- if rate has not end date, is the current period
or rate.effDateThru > w.workDate -- 'date-thru' represents the start date of the next period, so the this upper bound is EXCLUSIVE
)
we join the work table against a virtual rate table that gives us each employee's wage and the date range for which it is effective. The 'current' row for each employee will have the thru/expiry date set to null. And...since the thru/expiry date is actually the effective date for the next salary entry, the upper bound is exclusive rather than inclusive. Consequently, the range test must test for null and one can't use between.