Question

使用两个select语句的下面的查询有效：

create or replace view q2 as
select count(p.id) nstudents,
(select count(p.id) nstaff
from people p, staff s where p.id = s.id)
from people p, students s where p.id = s.id;

但是当我包含第三个子查询时：

create or replace view q2 as
select count(p.id) nstudents,
(select count(p.id) nstaff,
(select count(p.id) nboth
from people p, students s, staff t where p. id = s.id and p.id = t.id)
from people p, staff t where p.id = t.id)
from people p, students s where p.id = s.id;

它给了我以下错误：

ERROR:  subquery must return only one column
LINE 3: (select count(p.id) nstaff,

我在包含第三个查询时是否犯了一些错误，或者只有2个嵌套选择语句的限制？

Answer 1

带有附加子查询的版本失败了，因为您将第二个子查询插入第一个子查询时，它应该像这样：

select count(p.id) nstudents,
(select count(p.id) from people p, staff t where p.id = t.id) nstaff ,
(select count(p.id) from people p, students s, staff t where p. id = s.id and p.id = t.id) nboth 
from people p, students s where p.id = s.id;

但是，查询也可以使用条件聚合（以及使用显式ANSI标准连接）来编写，如下所示：

select 
    sum(case when s.id is not null then 1 end) nstudents,
    sum(case when t.id is not null then 1 end) nstaff,
    sum(case when s.id is not null and t.id is not null then 1 end) nboth
from people p
left join students s on p. id = s.id  
left join staff t on p.id = t.id

SQL - 使用ANSI标准

1 个答案: