使用 postgresql 的 GROUP BY CASE 语句

Question

I am trying to group by the CASE statement with postgresql, but i'm having this error "aggregate functions are not allowed in GROUP BY"

with zones as (select school, region, dt from district where dt between '1-jan-2021' and '31-jan-2021')
select c.region, to_char(a.dt, 'yyyy-mm') mth,
case when sum(a.payment)=0 then '0' else '1' end as seg,
sum(a.records)  records1,
sum(a.amount)  payment
from budget a   
left join zones c on (a.school = c.school and a.dt = c.dt)
where a.dt between '1-jan-2021' and '31-jan-2021'
group by   c.region, to_char(a.dt, 'yyyy-mm'), seg
order by c.site, to_char(a.dt, 'yyyy-mm'), seg;

thank you in advance

Answer 1

The result of a group by can only be the expressions with attributes in the group (and attributes that are functionally dependent on the attributes in the group by) and aggregation functions (the parms to these functions can be any attribute). I suspect you can rewrite your query such that the final function is the sum (move the case inside the sum). But...

Since the "case" is in the final projection, you can do first do the the group by, then do another projection.

Do this (make sure you give names to each attribute that is computed with a sum, such as sumpayment):

WITH t as (-- original query with case removed
    SELECT ..., sum(a.payment) as sumpayment, 
     ...)
select ..., case when sumpayment=0 then '0' else '1' end as seg, 
   ... -- any other attribute
from T

that should do it