MySQL：根据来自两个不同表的多个键选择几行

Question

我有这两张表——user_schedules和user_schedule_meta，如下图：

------------------------------------
| id | scheduler_id | status | 
------------------------------------
  1        3          pending 
  2        5          active
  3        6          active

和

----------------------------------------------
| id | user_schedule_id | meta_key |meta_value
----------------------------------------------
 1         3              course-id    135
 2         3              session-id   15
 3         3              schedule-id  120

我想编写一个查询，让我能够从两个表中选择，例如，满足以下 5 个条件中的每一个：

1. user_schedule_id = 3
2. scheduler_id = 6
3. session_id = 15
4. 课程 ID = 135
5. 计划 ID = 120

这是我目前所拥有的，但它不起作用：

SELECT user_schedule_meta.`id` FROM user_schedule_meta, user_schedules 
WHERE user_schedules.`scheduler_id` = 6 
AND user_schedules.id = user_schedule_meta.`user_schedule_id` 
AND ( 
(user_schedule_meta.`meta_key` = 'course-id' AND user_schedule_meta.`meta_value` = 135) 
OR (user_schedule_meta.`meta_key` = 'session-id' AND user_schedule_meta.`meta_value` = 15) 
OR (user_schedule_meta.`meta_key` = 'daily-schedule-id' AND user_schedule_meta.`meta_value` = 120) 
) 
GROUP BY user_schedule_meta.`id`

有什么建议我做的不对吗？

Answer 1

这是一个典型的键值存储查找问题。这些比在 SQL 中看起来更棘手，因为它们需要多个 JOIN 操作。

您需要一个每个user_schedules.id 值有一行的虚拟表，然后您可以对其进行过滤。所以

SELECT u.id, u.scheduler_id
  FROM user_schedules u
  JOIN user_schedule_meta a ON u.id=a.user_schedule_id AND a.meta_key='course-id'
  JOIN user_schedule_meta b ON u.id=b.user_schedule_id AND b.meta_key='session-id'
  JOIN user_schedule_meta c ON u.id=c.user_schedule_id AND c.meta_key='daily-schedule-id'

WHERE a.meta_value = 135  -- value associated with course-id
  AND b.meta_value=15     -- value associated with session-id
  AND c.meta_value=120    -- value associated with daily-schedule-id

还请注意，您可以像这样列出带有相关属性的表格。这种多次加入键/值表的技巧是一种 pivot 操作。我使用 LEFT JOIN 是因为它允许结果集显示缺少属性的行。

SELECT u.id, u.scheduler_id, u.status,
       a.meta_value AS course_id,
       b.meta_value AS session_id,
       c.meta_value AS daily_schedule_id
  FROM user_schedules u
  LEFT JOIN user_schedule_meta a ON u.id=a.user_schedule_id AND a.meta_key='course-id'
  LEFT JOIN user_schedule_meta b ON u.id=b.user_schedule_id AND b.meta_key='session-id'
  LEFT JOIN user_schedule_meta c ON u.id=c.user_schedule_id AND c.meta_key='daily-schedule-id'

Answer 2

试试这是代码

select * from user_schedule_meta where user_schedule_id=3 and 
(meta_key='session-id' AND meta_value=15
or meta_key='daily-schedule-id' AND meta_value=120
or meta_key='course-id' AND meta_value=135
)