SQL子查詢—sum()及減法

需求：

查詢 C 的結果，其中 C = A - B;

A的結果需要連表查詢；

B的結果也需要連表查詢；

實現：

過程一：

select sum(A) from 表a a
left join 表b b

on a.pid = b.id

where b.id = 1;

過程二：

select sum(B) from 表c c
left join 表d d
on c.pid = d.id

where d.id = 1;

結果：

select(m.A - n.B) as result
from
(select sum(A) as A from 表a a
left join 表b b
on a.pid = b.id
where a.id = 1) m,
(select sum(B) as B from 表c c
left join 表d d
on c.pid = d.id
where d.id = 1)n;