SQL如何在以下情况下联接两个表以获取记录？

Question

Payments

Year     Month       Division       Department   Payments_received_Count
------------------------------------------------------------------------
2016       1           Electric      dep1         2
2016       1           Electric      dep2         3
2015       1           Electric      dep1         1

Divisions

Division             Department   
--------------------------------
Electric             Dep1                      
Electric             Dep2                 
Electric             Dep3        

How to join the tables to get the following result?

Year     Month       Division       Department   Payments_received_Count
------------------------------------------------------------------------
2016       1           Electric      dep1         2
2016       1           Electric      dep2         3
2016       1           Electric      dep3         0
2015       1           Electric      dep1         1
2015       1           Electric      dep2         0
2015       1           Electric      dep3         0

So for each year and month I have to join the payments table with division table. Please suggest an approach.

I know CASE WHEN will help in this scenario, but I have not been able to find the right way.

CASE Payments_received_Count
     WHEN null THEN 0
     ELSE Payments_received_Count

Thanks

Answer 1

You need a combination of division/department and year/month. First, generate the rows using cross join . Then use left join to get the values:

select ym.year, ym.month, d.division, d.department,
       coalesce(Payments_received_Count, 0) as Payments_received_Count
from divisions d cross join
     (select distinct year, month from payments) ym left join
     payments p
     on d.division = p.division and d.department = p.department and
        ym.year = p.year and ym.month = p.month
order by year desc, month desc, division, department;