Question

i’m new here as I’m learning SQL and I’ve encountered a problem I can’t solve. Please help me.
I have two tables Outcome and Income with data, see screenshot of the tables.
https://www.box.com/s/5c1ah5qi9jg5ty499wvs

I want to join these tables but due to carthesian product some of the data is being added two times. Here is my code:

select o.point, o.date, sum(out), sum(inc) from outcome o left join income i on o.point=i.point and o.date=i.date 
group by o.point, o.date
union
select i.point, i.date, sum(out), sum(inc) from income i left join outcome o on o.point=i.point and o.date=i.date
group by i.point, i.date

Any advice? Thanks in advance.

G.

Answer 1

I think what you want is a full outer join rather than a union:

select coalesce(o.point, i.point) as point,
       coalesce(o.date, i.date) as date,
       sum(out), sum(inc)
from outcome o full outer join
     income i
     on o.point=i.point and o.date=i.date 
group by coalesce(o.point, i.point) , coalesce(o.date, i.date) 

Alternatively, you might want to do a union between the two tables, and then aggregate, as in:

select point, date, sum(out), sum(inc)
from ((select o.point, o.date, out, NULL as inc
       from outcome
      ) union all
      (select i.point, i.date, NULL as out, inc
       from income
      )
     ) io
group by point, date

This version will work correctly, even if there are multiple rows in each table for a given point/date combination.