PostgreSQL 不同并在不同字段上分组

Posted 2023-02-16

【中文标题】PostgreSQL 不同并在不同字段上分组

【英文标题】：PostgreSQL distinct and group on different fields

【发布时间】：2020-11-16 06:05:01

【问题描述】：

通过以下查询，我可以获得添加到成员资格表中的项目成员列表，并与项目所有者联合（他们可能在成员资格表中没有条目）

select sub.user, sub.project, sub.role, sub.order, sub.name from
(SELECT 
    memberships."user",
    memberships.project,
    memberships.role,
    roles."order",
    roles.name
FROM memberships
    JOIN roles ON roles.id = memberships.role
UNION
    SELECT projects.owner AS "user",
        projects.id AS project,
        1 AS role,
        0 AS "order",
        'admin'::text AS name
    FROM projects
) as sub

上述查询产生以下结果集。

8   2   1   0   "admin"
8   1   3   2   "contributor"       (added through memberships table)
6   1   1   0   "admin"
8   4   1   0   "admin"
8   1   1   0   "admin"             (duplicate because user #8 is the owner of project #1)

现在我想通过获取最少order 的行的内容来删除重复的条目。使用distinct on (sub.order) 不包括所有行

select distinct on (sub.order) * from
-- the same subquery
order by sub.order

以上产量

8   2   1   0   "admin"
8   1   3   2   "contributor"

使用group by sub.user, sub.project 和聚合min(sub.order) 有效，但其他两个字段如role 和name 被忽略

select sub.user, sub.project, min(sub.order) from
-- the same subquery
group by sub.user, sub.project

我想要role、name 和order 与user, project 对分组时具有最小order 的行

【参考方案1】：

我想要与用户、项目对分组时具有最小顺序的行的角色、名称和顺序

distinct on 必须枚举“分组”列 - 然后order by 子句必须包含相同的列，然后是用于打破联系的列。

你可能想要：

select distinct on (t.user, t.project) * 
from ( 
    -- the same subquery -- 
) t
order by t.user, t.project, t.order