SQL Statement Join With CASE

Question

I have problem with this 3 table below from MS ACCESS to do \"SQL CASE IF ELSE\" where I don\'t how to start.

Table A (Registrations)

Name    | Desc              


        

Answer 1

Here is the straight-forward solution with subqueries instead of aggregation and case constructs first. (I use NZ to return a 0 in case of NULL. This would be COALESCE in standard SQL).

select 
  name, 
  year,
  class,
  (
    select nz(sum(r.amount), 0)
    from registrations r 
    where r.name = s.name
    and r.year = s.year
    and r.desc = 'JAN&NOV'
  ) as "Jan&Nov",
  (
    select nz(sum(r.amount), 0)
    from monthly m 
    where m.name = s.name
    and m.year = s.year
    and desc = 'PAY FEB'
  ) as "Pay Feb",
  ...
from student_list s;

And here is the same with joins, aggregation and IIF (which would be CASE in standard SQL):

select 
  s.name, 
  s.year,
  s.class,
  nz(max(r.amount), 0) as "Jan&Nov",
  nz(max(iif(m.desc = 'PAY FEB', m.amount end, null)), 0) as "Pay Feb",
  ...
from student_list s
left join
(
  select 
    name, 
    year,
    sum(amount) as amount
  from registrations
  where desc = 'JAN&NOV'
  group by name, year
) r on r.name = s.name and r.year = s.year
left join
(
  select 
    name, 
    year,
    desc,
    sum(amount) as amount
  from monthly
  where group by name, year, desc
) m on m.name = s.name and m.year = s.year
group by s.name, s.year, s.class;

(You may or may not need additional parentheses around the joins. I think I remember MS Access to be rather peculiar with the join syntax.)

As to displaying the numbers in a certain format: this is something you'd usually do in your GUI layer. When using SQL for the formatting instead, you must convert the mere numbers into strings, e.g. 20 into '20.00'. In MS Access you do this with FORMAT:

format (value, 'Standard')

With the second query:

format(nz(max(iif(m.desc = 'PAY FEB', m.amount end, null)), 0), 'Standard')

which shows '0.00' in case of NULL. Or:

nz(format(max(iif(m.desc = 'PAY FEB', m.amount, null)), 'Standard'), 'NOT PAID')

which shows 'NOT PAID' in case of NULL.