PostgreSQL - Removing NULLS row and column from conditional aggregation results - sql

Question

I have a query for a multidimensional table using conditional aggregation
select A,
SUM(case when D = 3 then D end) as SUM_D1,
SUM(case when D = 4 then D end) as SUM_D2)
The result:
A SUM_D1 SUM_D2
-------------------
a1 100 NULL
a1 200 NULL
a3 NULL NULL
a4 NULL NULL
However, I would like to hide all NULL rows and columns as follows:
A SUM_D1
-----------
a1 100
a1 200
I have looked for similar problems but they are not my expected answer.
Any help is much appreciated,
Thank you

Answer 1

I think this does what you want:
select A,
coalesce(sum(case when D = 3 then D end),
sum(case when D = 4 then D end)
) as sum_d
from t
group by A
having sum(case when d in (3, 4) then 1 else 0 end) > 0;
Note that this returns only one column -- as in your example. If both "3" and "4" are in the data, then the value is for the "3"s.
If you want a query that returns a variable number of columns, then you need to use dynamic SQL -- or some other method. SQL queries return a fixed number of columns.
One method would be to return the values as an array:
select a,
array_agg(d order by d) as ds,
array_agg(sumd order by d) as sumds
from (select a, d, sum(d) as sumd
from t
where d in (3, 4)
group by a, d
) d
group by a;

Answer 2

To filter all-NULL rows you can use HAVING
select *
from
(
select A,
SUM(case when D = 3 then D end) as SUM_D1,
SUM(case when D = 4 then D end) as SUM_D2)
...
) as dt
where SUM_D1 is not null
and SUM_D2 is not null
Of course, if you got simple conditions like the ones in your example you better filter before aggregation:
select A,
SUM(case when D = 3 then D end) as SUM_D1,
SUM(case when D = 4 then D end) as SUM_D2)
...
where D in (3,4)
Now at least one calculation will return a value, thus no need to check for all-NULL.
To filter all-NULL columns you need some Dynamic SQL:
materialize the data in a temporary tabke using Insert/Select
scan each column for all-NULL select 1 from temp having count(SUM_D1) > 0
dynamically create the Select list based on this
run the Select
But why do you think you need this? It will be confusing for a user to run the same Stored Procedure and receive a different number of columns for each run.

Answer 3

I may have misinterpreted your question because the solution seems so simple:
select A,
SUM(case when D = 3 then D end) as SUM_D1,
SUM(case when D = 4 then D end) as SUM_D2)
where D is not null
This is not what you want, is it? :-)

Answer 4

Null appear because the condition that's not handled by case statement
select A,
SUM(case when D = 3 then D end) as SUM_D1,
SUM(case when D = 4 then D end) as SUM_D2
from
Table1
group by
A
having
(case when D = 3 or D = 4 then D end) is not null
As comment said if you want to suppress the null value.. You can use having to suppress null using is not null