mysql - 选择用户并根据相关表中存在的行进行透视

Question

我是 sql 新手，我真的试图自己解决这个问题，但没有成功......希望有人能帮忙。

我有 3 个表:

授权用户

+--------------+--------------+------+-----+---------+----------------+
| Field        | Type         | Null | Key | Default | Extra          |
+--------------+--------------+------+-----+---------+----------------+
| id           | int(11)      | NO   | PRI | NULL    | auto_increment |
| username     | varchar(30)  | NO   | UNI | NULL    |                |
| first_name   | varchar(30)  | NO   |     | NULL    |                |
| last_name    | varchar(30)  | NO   |     | NULL    |                |
| email        | varchar(75)  | NO   |     | NULL    |                |
| password     | varchar(128) | NO   |     | NULL    |                |
| is_staff     | tinyint(1)   | NO   |     | NULL    |                |
| is_active    | tinyint(1)   | NO   |     | NULL    |                |
| is_superuser | tinyint(1)   | NO   |     | NULL    |                |
| last_login   | datetime     | NO   |     | NULL    |                |
| date_joined  | datetime     | NO   |     | NULL    |                |
+--------------+--------------+------+-----+---------+----------------+

目录礼物

+-------------+----------------------+------+-----+---------+----------------+
| Field       | Type                 | Null | Key | Default | Extra          |
+-------------+----------------------+------+-----+---------+----------------+
| id          | int(11)              | NO   | PRI | NULL    | auto_increment |
| name        | varchar(48)          | NO   |     | NULL    |                |
| slug        | varchar(50)          | NO   | UNI | NULL    |                |
| amount      | smallint(5) unsigned | NO   |     | 0       |                |
| points      | smallint(5) unsigned | NO   |     | 500     |                |
| created_at  | datetime             | NO   |     | NULL    |                |
| updated_at  | datetime             | NO   |     | NULL    |                |
| active      | tinyint(1)           | NO   |     | NULL    |                |
| image       | varchar(100)         | YES  |     | NULL    |                |
| description | longtext             | YES  |     | NULL    |                |
+-------------+----------------------+------+-----+---------+----------------+

目录订单

+------------+----------------------+------+-----+---------+----------------+
| Field      | Type                 | Null | Key | Default | Extra          |
+------------+----------------------+------+-----+---------+----------------+
| id         | int(11)              | NO   | PRI | NULL    | auto_increment |
| user_id    | int(11)              | NO   | MUL | NULL    |                |
| status     | smallint(5) unsigned | NO   |     | 0       |                |
| present_id | int(11)              | NO   | MUL | NULL    |                |
| address_id | int(11)              | NO   | MUL | NULL    |                |
| created_at | datetime             | NO   |     | NULL    |                |
| updated_at | datetime             | NO   |     | NULL    |                |
+------------+----------------------+------+-----+---------+----------------+

我正在尝试选择一个用户并检查他是否订购了特定的礼物并相应地写入该行，然后检查另一个，然后再检查另一个等等。

基本上我希望结果看起来像这样:

id present1 present2 present3 present4
1 1 0 0
2 1 1 1
3 0 0 0
4 1 0 1

其中 1 是用户订购了该类型的礼物，0 是他没有。

我的查询是这样的

select auth_user.id, case when present_id = 1 from auth_user 
left join catalog_orders on catalog_orders.user_id = auth_user.id 
left join catalog_presents on catalog_presents.id = catalog_orders.present_id

问题是用户的所有不同订单都像这样在多行中排序:

id present1 present2 present3 present4
1 1 0 0
2 1 0 0
2 0 1 0
2 0 0 1
3 0 0 0
4 1 0 0
4 0 0 1

有人可以帮我解决这个问题吗？提前致谢!

Answer 1

这是一种数据透视表。

您的方向是正确的，但您需要将结果合并为每个用户一行。

每个 auth_user.id 使用聚合 MAX() 将它们折叠成一行。 CASE 为每个存在提供一个零或一个，然后 MAX() 为 auth_user.id 选择所有行的最大值，如果礼物已购买，则为 1，否则为 0。

SELECT
  auth_user.id,
  MAX(CASE WHEN present_id = 1 THEN 1 ELSE 0 END) AS present1,
  MAX(CASE WHEN present_id = 2 THEN 1 ELSE 0 END) AS present2,
  MAX(CASE WHEN present_id = 3 THEN 1 ELSE 0 END) AS present3,
  MAX(CASE WHEN present_id = 4 THEN 1 ELSE 0 END) AS present4
FROM
   auth_user
   LEFT JOIN catalog_orders on catalog_orders.user_id = auth_user.id 
   LEFT JOIN catalog_presents on catalog_presents.id = catalog_orders.present_id
GROUP BY auth_user.id
ORDER BY auth_user.id

特别是对于 MySQL，您不需要 CASE，因为 bool 比较 present_id = 1 将自行返回 1 或 0。但是，这不能移植到所有其他 RDBMS。优先采用上述方法。

SELECT
  auth_user.id,
  /* MySQL ok, not all other RDBMS will do this - boolean comparison returns 1 or 0 */
  MAX(present_id = 1) AS present1,
  MAX(present_id = 2) AS present2,
  MAX(present_id = 3) AS present3,
  MAX(present_id = 4) AS present4
FROM
   auth_user
   LEFT JOIN catalog_orders on catalog_orders.user_id = auth_user.id 
   LEFT JOIN catalog_presents on catalog_presents.id = catalog_orders.present_id
GROUP BY auth_user.id
ORDER BY auth_user.id