Question

我正在一所在线大学工作，该大学拥有用户，课程和用户到课程的状态。我有一个用户列表和另一个课程列表。我想查找所有给定用户和课程的课程状态，包括用户尚未开始的课程的空状态。

例如：

User IDs: [1, 7, 14, 21]  
Course IDs: [5, 8, 36, 50]

期望的结果：

Name        Course                     Status
 John Doe    How to Tie your Shoes      Complete
 John Doe    How to Paint your House    In Progress
 Jane Doe    How to Tie your Shoes      Complete
 Jane Doe    How to Paint your House    Not Started <-- These are the tricky ones

...

似乎我可以在表上执行LEFT JOIN并获得一些NULL值，我可以将其合并到“未开始”状态。但是只要我添加一些约束来限制我正在寻找的课程和/或用户......它就会停止给我NULL值，因为我的课程列表中的NULL课程ID显然不是

以下是一个示例查询，让您了解我一直在尝试的内容（以及其他内容）：

SELECT
    `users`.`name` AS `Name`,
    `users`.`email` AS `Email`,
    `courses`.`number` AS `Course #`,
    `courses`.`name` AS `Course`,
    COALESCE(`courses_users_statuses`.`name`, 'Not Started') AS `Status`
FROM
    `users`
    LEFT JOIN `courses_users` 
        ON `courses_users`.`user_id` = `users`.`id`
    LEFT JOIN `courses` 
        ON `courses`.`id` = `courses_users`.`course_id`
    LEFT JOIN `courses_users_statuses` 
        ON `courses_users_statuses`.`id` = `courses_users`.`status_id`
WHERE
    `courses`.`id` IN ([1, 2, 3, 4, 5, 10, 11, 12, 16, ...])
    AND `users`.`id` IN ([1, 2, 3, 4, 5, 20, 21, 36, 48, ...])
ORDER BY
    `users`.`name`,
    `courses`.`number`

关于如何写这样的东西的任何想法？另外，如果我能提供更多详细信息或更多代码/表格示例，请与我们联系。

编辑：这是我的更新查询，使用以下答案中的建议：

SELECT
    `users`.`name` AS `Name`,
    `users`.`email` AS `Email`,
    `courses`.`number` AS `Course #`,
    `courses`.`name` AS `Course`,
    COALESCE(`courses_users_statuses`.`name`, 'Not Started') AS `Status`
FROM
    `users`
        LEFT JOIN
    `courses_users` ON `courses_users`.`user_id` = `users`.`id`
        LEFT JOIN
    `courses` ON `courses`.`id` = `courses_users`.`course_id` AND `courses`.`id` IN (1, 2, 3, 4, 5)
        LEFT JOIN
    `courses_users_statuses` ON `courses_users_statuses`.`id` = `courses_users`.`status_id`
WHERE
    `users`.`id` IN (1, 2, 3, 4, 5)
ORDER BY
    `partners`.`name`,
    `users`.`name`,
    `courses`.`number`

这个更新的例子是一个改进，但现在它显示没有课程名称或编号的记录，但是有一个状态。我不确定它是如何抓住应该存在的课程关系的状态的。相反，那些应该是NULL（或＆＃34;未开始＆＃34;）。这是来自数据库的一些示例数据：

`users`表：

id    name             email
1     Stevie McComb    test@example.com
2     John Doe         test@example.org
3     Jane Doe         test@example.net

`courses`表：

id    number    name
1     101       Navigation
2     102       Logging In
3     103       Updating Records
4     104       Managing Users

`courses_users`表：

course_id    user_id    status_id    completed_at
1            1          2            2017-01-01 00:00:00
3            1          1            2017-01-05 00:23:00
1            2          2            2017-04-13 15:00:37

`courses_users_statuses`表：

id    name           slug
1     In Progress    progress
2     Complete       complete

期望的结果：

Name             Email               Course #    Course              Status
Stevie McComb    test@example.com    101         Navigation          Complete
Stevie McComb    test@example.com    102         Logging In          Not Started
Stevie McComb    test@example.com    103         Updating Records    In Progress
Stevie McComb    test@example.com    104         Managing Users      Not Started
John Doe         test@example.org    101         Navigation          Complete
John Doe         test@example.org    102         Logging In          Not Started
John Doe         test@example.org    103         Updating Records    Not Started
John Doe         test@example.org    104         Managing Users      Not Started
Jane Doe         test@example.net    101         Navigation          Not Started
Jane Doe         test@example.net    102         Logging In          Not Started
Jane Doe         test@example.net    103         Updating Records    Not Started
Jane Doe         test@example.net    104         Managing Users      Not Started

当前结果：

Name             Email               Course #    Course              Status
Stevie McComb    test@example.com                                    Complete
Stevie McComb    test@example.com                                    Not Started
Stevie McComb    test@example.com    103         Updating Records    In Progress
Stevie McComb    test@example.com                                    Not Started
John Doe         test@example.org    101         Navigation          Complete
John Doe         test@example.org                                    Not Started
John Doe         test@example.org                                    Not Started
John Doe         test@example.org                                    Not Started
Jane Doe         test@example.net                                    Not Started
Jane Doe         test@example.net                                    Not Started
Jane Doe         test@example.net                                    Not Started
Jane Doe         test@example.net                                    Not Started

Answer 1

问题在于您将约束放在where子句中，而不允许null值。这实际上会将left join更改回inner join。

要解决此问题，您可以显式允许空值，也可以将逻辑移动到join子句（我的首选项）。

select
    `users`.`name` as `name`,
    `users`.`email` as `email`,
    `courses`.`number` as `course #`,
    `courses`.`name` as `course`,
    coalesce(`courses_users_statuses`.`name`, 'not started') as `status`
from
    `users`
    left join `courses_users` 
        on `courses_users`.`user_id` = `users`.`id`
    left join `courses` 
        on `courses`.`id`   = `courses_users`.`course_id`
        and `courses`.`id` in ([1, 2, 3, 4, 5, 10, 11, 12, 16, ...])
    left join `courses_users_statuses` 
        on `courses_users_statuses`.`id` = `courses_users`.`status_id`
where
    `users`.`id` in ([1, 2, 3, 4, 5, 20, 21, 36, 48, ...])
order by
    `users`.`name`,
    `courses`.`number`

Answer 2

当我必须处理可能的NULL值时，我使用

IFNULL(field_name, 'use this value instead');

例如

SELECT
IFNULL(Course, 'Nothing Found') AS course
FROM
Course
Where....

或者您可以在WHERE子句中指定...

WHERE Course IS NOT NULL

MySQL查询用于查找具有约束的三个表之间的关系，即使该关系不存在

2 个答案: