PostgreSQL结合了多个select语句

Question

我使用Excel SELECT从具有大量相同模式的数据库中的模式名称列表中生成大量语句:

select result from foo.table limit 1;
select result from bar.table limit 1;
select result from doo.table limit 1;

(foo,bar&doo是我的架构的例子,现实中有数百个).

每个SELECT只返回一个结果.我只想要一个列result与模式一样多的行.然后我可以根据模式名称将其复制回Excel.

当我运行上面的查询时,我得到1行,其他行被丢弃:

Query result with 1 row discarded.

Query result with 1 row discarded.

Total query runtime: 40 ms.
1 row retrieved.

我尝试过使用UNION ALL,但是limit 1我用来确保每个模式表只返回一行,以防止这种情况发生.

如何阻止其他行被丢弃,或者以更有效的方式编写将返回我需要的值的查询(两列 - schema_name,结果 - 每个模式一行)？

Answer 1

只需在括号中包含单个语句,以使语法明确:

(SELECT result FROM tbl1 LIMIT 1)
UNION ALL
(SELECT result FROM tbl2 LIMIT 1)

关于UNION此事的手册非常明确:

select_statement任何SELECT没有声明ORDER BY,LIMIT, FOR UPDATE,或FOR SHARE条款.(ORDER BY并且LIMIT可以,如果它被包围在括号被附着到子表达式,没有括号,这些条款将被视为适用于的结果 UNION,而不是它的右手输入表达式.)

Answer 2

create view my_data1 
AS
with data as
(
    select student_id,sum(marks) as total_marks 
    from   marks_marks 
    group by 1
) , 
data1 as
(
    select id, name 
    from   students_class
), 
data2 as
(
    select applicant_name,
           id,
           class_name 
    from   students_students
)
select data2.applicant_name , 
       data1.name as class_name ,  
       data.total_marks 
from   data2 
join   data1  on data1.id = data2.class_name 
join   data   on data.student_id = data2.id



select * from my_data1

Answer 3

在子查询中包装会绕过它,但它有点难看.

SELECT result FROM (select 'a'::text AS result from foo limit 1) a
UNION ALL
SELECT result FROM (select 'b'::text AS result from bar limit 1) b

UPDATE

见欧文的回应.这个比较好.