PostgreSQL distinct and group on different fields

Question

通过以下查询，我可以获得在成员资格 table 中添加的项目成员列表，与项目所有者联合（他们可能在成员资格中没有条目 table )

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

以上结果

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

使用 group by sub.user, sub.project 并聚合 min(sub.order) 有效，但其他两个字段如 role 和 name 被排除在外

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[=26= 分组时具有最小 order 的行]

Answer 1

I want the role, name and order of the row that has the minimum order when grouped with user, project pair

distinct on 必须枚举“分组”列 - 然后 order by 子句必须包含相同的列，后跟用于打破关系的列。

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

PostgreSQL distinct and group on different fields

PostgreSQL distinct and group on different fields

postgresql

union

join

group-by

greatest-n-per-group