与其他case语句一起使用时，CASE表达式返回NULL，在单独时返回预期结果

Question

When i run this line of code alone i get the expected result of 1:

sum(case when facilityname like '%AT%' then count(status) else 0 end) as AT_all_status

However when i run with multiple case statements together as shown in this example all case statements return NULL:

sum(case when facilityname like '%AT%' then count(status) else 0 end) as AT_all_status,
sum(case when facilityname like '%AT%' and status in ('Current','Approved') then count(status) else 0 end) as AT_approved_current,
sum(case when facilityname like '%CZ%' then count(status) else 0 end) as CZ_all_status,
sum(case when facilityname like '%CZ%' and status in ('Current','Approved') then count(status) else 0 end) as CZ_approved_current,
sum(case when facilityname like '%FGE%' then count(status) else 0 end) as FGE_all_status,
sum(case when facilityname like '%FGE%' and status in ('Current','Approved') then count(status) else 0 end) as FGE_approved_current,
sum(case when facilityname like '%FRA%' then count(status) else 0 end) as FRA_all_status,
sum(case when facilityname like '%FRA%' and status in ('Current','Approved') then count(status) else 0 end) as FRA_approved_current

When they should actually be returning their own numbers, and do so when ran alone, is there a case statement behaviour i'm missing here?

Answer 1

In standard SQL, this should not return any results (other than an error):

sum(case when facilityname like '%AT%' then count(status) else 0 end) as AT_all_status

because aggregation functions cannot be nested. Oracle does allow nesting them, but I don't commend using that functionality -- a subquery is simple enough.

In any case, I'm pretty sure that you actually intend:

sum(case when facilityname like '%AT%' then 1 else 0 end) as AT_all_status

Answer 2

仅使用一个聚合函数而不嵌套的翻译查询的另一种方法是：

count(case when facilityname like '%AT%' then status end)