在 postgres 选择中,将列子查询作为数组返回? [英] in postgres select, return a column subquery as an array?
问题描述
(以前也这样做过,但是记忆会消失,就像护目镜一样)
希望从 users
中选择每个用户的 tag.tag_id
s 作为数组返回.
使用嵌入查询的想法 tag_arr
将是一个数组
使用 聚合函数:
选择usr_id,名称,array_agg(tag_id) 作为 tag_arr来自用户使用(usr_id)加入标签按 usr_id、名称分组
或 数组构造函数 来自子查询的结果:
选择u.usr_id,名称,大批(选择 tag_id从标签 t其中 t.usr_id = u.usr_id) 作为 tag_arr来自用户
第二个选项是简单的单源查询,而第一个更通用,当您需要来自相关表的多个聚合时特别方便.此外,第一个变体在较大的表上应该更快.
注意,为了更好的性能,两个表中的 usr_id
列都应该被索引.虽然通常 users.usr_id
是主键,但有时可能会忘记引用列的索引也很有用.
(have done this before, but memory fades, as does goggle)
wish to get select from users
with the tag.tag_id
s for each user returned as an array.
select usr_id, name, (select t.tag_id from tags t where t.usr_id = u.usr_id) as tag_arr from users u;
with the idea embedded query tag_arr
would be an array
Use the aggregate function:
select
usr_id,
name,
array_agg(tag_id) as tag_arr
from users
join tags using(usr_id)
group by usr_id, name
or an array constructor from the results of a subquery:
select
u.usr_id,
name,
array(
select tag_id
from tags t
where t.usr_id = u.usr_id
) as tag_arr
from users u
The second option is a simple one-source query while the first one is more generic, especially convenient when you need more than one aggregate from a related table. Also, the first variant should be faster on larger tables.
Note, that for better performance the usr_id
columns in both tables should be indexed. While typically users.usr_id
is a primary key, sometimes one may forget that the index of referencing column is also useful.
这篇关于在 postgres 选择中,将列子查询作为数组返回?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!