如何在postgresql中声明变量并获取结果集?

像这样使用两个子选择

select (select count(*) cnt_male from tbl where gender='M') 
      ,(select count(*) cnt_female from tbl where gender='F')

或

您可以使用 case 在里面 count() 功能如下

select count(case when gender='M' then 1 end) cnt_male
      ,count(case when gender='F' then 1 end) cnt_female 
from tbl

结果:

cnt_male cnt_female 
-------- ---------- 
2        2          

编辑:

根据操作 update

create function fn_gender_cnt() returns table(tot_male bigint,tot_female bigint)
as
$$

  select count(case when gender='M' then 1 end) cnt_male
      ,count(case when gender='F' then 1 end) cnt_female 
from tbl

$$
language sql

用法:

select * from fn_gender_cnt();

结果:

    tot_male tot_female 
    -------- ---------- 
    2        2