我的目标是编写一个接受一个参数并返回两个值的函数。查询运行得很好,但是,当通过make函数执行时,我收到一个错误,即子查询不应返回多列。
我的功能如下:
CREATE TYPE double_integer_type AS (p1 integer, p2 integer);
DROP FUNCTION next_dvd_in_queue;
CREATE OR REPLACE FUNCTION next_dvd_in_queue (member_id_p1 integer) RETURNS double_integer_type as $$
BEGIN
RETURN(
select temp2.dvdid,
temp2.movie_title
from
(select temp1.dvdid,
temp1.movie_title,
temp1.customer_priority
from
(select *
from rentalqueue
where rentalqueue.memberid=member_id_p1) temp1
inner join dvd on dvd.dvdid=temp1.dvdid
where dvd.dvdquantityonhand>0) temp2
order by temp2.customer_priority asc
limit 1
);
END; $$ LANGUAGE PLPGSQL
呼叫:
select dvdid from next_dvd_in_queue(3);
当使用硬编码值执行时,查询为:
select temp2.dvdid,
temp2.movie_title
from
(select temp1.dvdid,
temp1.movie_title,
temp1.customer_priority
from
(select *
from rentalqueue
where rentalqueue.memberid=3) temp1
inner join dvd on dvd.dvdid=temp1.dvdid
where dvd.dvdquantityonhand>0) temp2
order by temp2.customer_priority asc
limit 1
上面的查询运行良好。
然而,当我以以下方式调用函数时:
select * from next_dvd_in_queue(3);
我得到以下错误:
ERROR: subquery must return only one column LINE 1: SELECT ( ^ QUERY: SELECT ( select temp2.dvdid, temp2.movie_title from (select temp1.dvdid, temp1.movie_title, temp1.customer_priority from (select * from rentalqueue where rentalqueue.memberid=3) temp1 inner join dvd on dvd.dvdid=temp1.dvdid where dvd.dvdquantityonhand>0) temp2 order by temp2.customer_priority asc limit 1 ) CONTEXT: PL/pgSQL function next_dvd_in_queue(integer) line 3 at RETURN
您可以通过显式转换为复合类型来修复语法错误:
CREATE OR REPLACE FUNCTION next_dvd_in_queue (member_id_p1 integer)
RETURNS double_integer_type AS
$func$
BEGIN
RETURN (
SELECTROW(temp2.dvdid, temp2.movie_title)::double_integer_type
FROM ...
);
END
$func$ LANGUAGE plpgsql
但我会消除复合类型不必要的复杂性,并使用OUT
参数:
CREATE OR REPLACE FUNCTION pg_temp.next_dvd_in_queue (member_id_p1 integer
OUT p1 integer
OUT p2 varchar(100)) AS
$func$
BEGIN
SELECT INTO p1, p2
temp2.dvdid, temp2.movie_title
FROM ...
END
$func$ LANGUAGE plpgsql;
避免参数名称和列名之间的命名冲突。我喜欢坚持一种命名约定,在这里我用_
作为所有参数名称的前缀,所以_member_id_p1
、_p1
、_p2
。
相关:
- 从带有OUT参数的函数返回
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