如何为添加到选择的子查询返回几列？

Question

我有一个这样的查询：

select 
   e.field1,
   e.field2,
   (select count(field3) from tbl1 where someField = e.field1 group By someType ) as count_1,
   (select count(field4) from tbl1 where someField = e.field1 group By someType ) as count_2,
from
...

我不喜欢代码重复计数，但如果我这样做的话

select count(field3), count(field3) ....

在子查询中 postgres 抱怨子查询必须返回单列。

如何解决？

Answer 1

您可以使用横向连接：

select e.field1,  e.field2, t1.*
from e left join lateral
     (seclect count(tbl1.field3) as count_1, count(tbl1.field4) as count_4
      from tbl1
      where tbl1.someField = e.field1
     ) t1
     on true;

您也可以通过聚合来做到这一点：

select e.field1,  e.field2, t1.count_1, t1.count_2
from e left join
     (select tbl1.someField, count(tbl1.field3) as count_1, count(tbl1.field4) as count_2
      from tbl1
      group by tbl1.someField 
     ) t1
     on tbl1.someField = e.field1;

这可能会返回 NULL 值，如果需要，您可以使用 coalesce() 将其转换为 0。