删除行重复并使用 PostgreSQL 获得最高值

Question

使用以下查询

SELECT
  users.id,
  users.username,
  users.email,
  groups.permission
FROM users
INNER JOIN memberships ON users.id = memberships.user_id
INNER JOIN groups ON memberships.group_id = groups.id
INNER JOIN groupings ON groups.id = groupings.group_id
WHERE groupings.repo_id = 1
GROUP BY users.id, groups.permission
ORDER BY users.id

我得到以下行：

----+---------------------------+-----------------------------------------+------------
 id |         username          |                  email                  | permission 
----+---------------------------+-----------------------------------------+------------
  2 | viva_leuschke0            | viva_leuschke0@ritchiealtenwerth.org    | 1
  2 | viva_leuschke0            | viva_leuschke0@ritchiealtenwerth.org    | 2
  3 | loyce_herman1             | herman1.loyce@bednar.com                | 1
  3 | loyce_herman1             | herman1.loyce@bednar.com                | 3
  4 | verona_vandervort2        | verona.vandervort2@mante.biz            | 1
  4 | verona_vandervort2        | verona.vandervort2@mante.biz            | 2
  4 | verona_vandervort2        | verona.vandervort2@mante.biz            | 3
  5 | bruen3_ms_hans            | ms.bruen3.hans@bechtelar.net            | 1
  5 | bruen3_ms_hans            | ms.bruen3.hans@bechtelar.net            | 2
  5 | bruen3_ms_hans            | ms.bruen3.hans@bechtelar.net            | 3
----+---------------------------+-----------------------------------------+------------

问题是：如何调整此查询以返回唯一行，按最高权限值过滤？比如：

----+---------------------------+-----------------------------------------+------------
 id |         username          |                  email                  | permission 
----+---------------------------+-----------------------------------------+------------
  2 | viva_leuschke0            | viva_leuschke0@ritchiealtenwerth.org    | 2
  3 | loyce_herman1             | herman1.loyce@bednar.com                | 3
  4 | verona_vandervort2        | verona.vandervort2@mante.biz            | 3
  5 | bruen3_ms_hans            | ms.bruen3.hans@bechtelar.net            | 3
----+---------------------------+-----------------------------------------+------------

我使用的是 PostgreSQL 9.1。

更新：通过使用DISTINCT ON 子句，我能够得到我想要的。

SELECT DISTINCT ON(users.id)
  users.id,
  users.username,
  users.email,
  groups.permission
FROM users
INNER JOIN memberships ON users.id = memberships.user_id
INNER JOIN groups ON memberships.group_id = groups.id
INNER JOIN groupings ON groups.id = groupings.group_id
WHERE groupings.repo_id = 1
GROUP BY users.id, groups.permission
ORDER BY 
  users.id ASC, 
  groups.permission DESC

这是最好的方法吗？

Answer 1

仔细检查后，您的查询可以（并且应该）改进：

SELECT DISTINCT ON (u.id)
      ,u.id
      ,u.username
      ,u.email
      ,g.permission
FROM   users       u
JOIN   memberships m  ON m.user_id = u.id
JOIN   groups      g  ON g.id = m.group_id
JOIN   groupings   gi ON gi.group_id = g.id 
WHERE  gi.repo_id = 1
GROUP  BY u.id, g.permission
ORDER  BY u.id, g.permission DESC

GROUP BY 没用。仅DISTINCT ON 就为您提供了独特的users.id。去掉GROUP BY，你会得到同样的结果，只是更快。

您可能希望向ORDER BY 添加更多列，以决定选择哪一行，其中每个id 有多个共享最高permission。您拥有它的方式，您可以随意选择这种情况（前提是它可以发生）。

My reference answer for this query technique has more explanation, a benchmark and links.