"UNION 与 CTE 一起工作，但不是没有？"

Question

I'm using PostgreSQL 11.12.

select first_name, length(first_name)
from db_employee
where length(first_name) = 
(select max(length(first_name)) from db_employee)
order by 1 
limit 1
union
select first_name, length(first_name)
from db_employee
where length(first_name) = 
(select min(length(first_name)) from db_employee)
order by 1 
limit 1

Answer 1

To include a LIMIT<\/code> clause per SELECT<\/code> in a UNION<\/code> query, you must add parentheses<\/strong> . Like:

(  -- !
SELECT first_name, length(first_name)
FROM   db_employee
WHERE  length(first_name) = (SELECT max(length(first_name)) FROM db_employee)
ORDER  BY 1
LIMIT  1
) -- !
UNION
( -- !
SELECT first_name, length(first_name)
FROM   db_employee
WHERE  length(first_name) = (SELECT min(length(first_name)) FROM db_employee)
ORDER  BY 1
LIMIT  1
) -- !

Answer 2

just put both the sql parts in separate brackets and keep union outside.

    (select first_name, length(first_name)
    from db_employee
    where length(first_name) = 
    (select max(length(first_name)) from db_employee)
    order by 1
    limit 1)
    union
    (select first_name, length(first_name)
    from db_employee
    where length(first_name) = 
    (select min(length(first_name)) from db_employee)
    order by 1
    limit 1);