SQL（DB2）在单个查询中返回多个条件计数

Question

I am trying select multiple conditional summaries into a single table result on a DB2 based database.

Example:

SELECT COUNT(FOO) FROM T1 WHERE T1.A=1 AS A_COUNT,
SELECT COUNT(FOO) FROM T1 WHERE T1.B=2 AS B_COUNT
Ext...

Any help is appreciated.

Answer 1

This will count the occurrence of each condition:

select sum(case when t1.a = 1 then 1 else 0 end) as A_COUNT
     , sum(case when t1.b = 2 then 1 else 0 end) as B_COUNT
  from t1
 where t1.a = 1
    or t1.b = 2

Answer 2

select count(case when t1.a = 1 then foo else null end) as A_COUNT
     , count(case when t1.b = 2 then foo else null end) as B_COUNT
  from t1
 where t1.a = 1
    or t1.b = 2

Where clause is optional strictly speaking but may assist performance. Also "else null" is implicit when the else clause is omitted so you can safely leave that off as well.

Answer 3

select count(foo)
  from t1
 where a = 1
union
select count(foo)
  from t1
 where b = 2
....

Answer 4

This will do it.

SELECT  A_COUNT as Type ,COUNT(FOO) FROM T1 WHERE T1.A=1, 

Union


SELECT B_COUNT as Type, COUNT(FOO) FROM T1 WHERE T1.B=2