用于查找 friend 签到的 SQL 查询

Question

我在 PostgreSQL 中有以下表格:

Categories | Locations   | Checkins     | Users  | Friendships
id         | id          | id           | id     | user_id
name       | category_id | location_id  | gender | friend_id
icon       | name        | user_id      |        |

现在，我想检索有关 field 的以下信息

• 一个地点有多少女性和男性用户
• 类别名称和图标
• 地点名称
• 有多少 friend 在某个位置签到(来自给定的用户 ID)

除了最后一点，我解决了。但是我很难从给定的用户 ID 中计算出 friend 。我用这个查询试过了:

SELECT distinct locations.id,
 max(locations.name) as name,
 max(locations.location) as location,
 max(categories.name) as cat,
 max(categories.icon) as caticon,
 SUM(CASE WHEN users.gender = 'm' THEN 1 ELSE 0 END) AS male,
 SUM(CASE WHEN users.gender = 'f' THEN 1 ELSE 0 END) AS female,
 SUM(CASE WHEN friendships.user_id = 1 OR friendships.friend_id=1 THEN 1 ELSE 0 END) AS friends
 FROM locations
 INNER JOIN checkins ON checkins.location_id = locations.id 
 INNER JOIN users ON users.id = checkins.user_id 
 INNER JOIN categories ON categories.id = locations.category_id
 LEFT JOIN friendships ON friendships.user_id = users.id OR friendships.friend_id = users.id
 WHERE locations.id=7
 GROUP BY locations.id

但我对女性用户的计数有误。知道我做错了什么吗？我想我需要为 friendships 表添加一个左连接，因为如果用户没有 friend (或没有给出用户)，它应该只为 friend 数返回 0。

希望我说清楚了，谢谢，晚礼服

Answer 1

SELECT
  L.id,
  L.name,
  c.name AS cat,
  c.icon AS caticon,
  COUNT(CASE u.gender WHEN 'm' THEN 1 END) AS male,
  COUNT(CASE u.gender WHEN 'f' THEN 1 END) AS female,
  COUNT(f.user_id) AS friends
FROM Locations L
  INNER JOIN Categories c ON c.id = L.category_id
  INNER JOIN Checkins ch ON ch.location_id = L.id
  INNER JOIN Users u ON u.id = ch.user_id
  LEFT JOIN Friendships f ON f.user_id = @user_id AND f.friend_id = ch.user_id
                          OR f.user_id = ch.user_id AND f.friend_id = @user_id
WHERE L.id = @location_id
GROUP BY L.id, L.name, c.name, c.icon