Question

I have a salary table like this:

id | person_id | start_date  | pay
1  | 1234      | 2012-01-01  | 3000
2  | 1234      | 2012-05-01  | 3500
3  | 5678      | 2012-01-01  | 5000
4  | 5678      | 2013-01-01  | 6000
5  | 9101      | 2012-09-01  | 2000
6  | 9101      | 2014-04-01  | 3000
7  | 9101      | 2011-01-01  | 1500
and so on...

Now I want to query the sum of the salaries of a specific month for all persons of a company.

I already have the ids of the persons who worked in the specific month in the specific company, so I can do something like WHERE person_id IN (…)

I have some problems with the salaries query though. The result for e.g. the month 2012-08 should be:

10000

which is 3500+5000+1500.

So I need to find the summed up pay value (for all persons in the IN clause) for the maximum start_date <= the specific month.

I tried various INNER JOINS but it’s been a long day and I can’t think straight at the moment.

Any hint is highly appreciated.

Answer 1

You need to get the active record. This following does this by calculating the max start date before the month in question:

select sum(s.pay)
from (select person_id, max(start_date) as maxstartdate
      from salary
      where person_id in ( . . . ) and
            start_date < <first day of month of interest>
      group by person_id
   ) p join
   salary s
   on s.person_id = p.person_id and
      s.maxstartdate = p.start_date

You need to fill in the month and list of ids.

You can also do this with ranking functions, but you don’t specify which SQL engine you are using.