Question

我有下表，表1显示所有用户，表2显示教师与学生的关系：

表1（用户）：

ID  name  age
-------------
1   Al    30
2   Bob   5
3   Cam   6
4   Dan   7

Table2（类）：

teacher_id  student_id 
----------------------
1           2
1           3

从本质上讲，这表明Al（id = 1）是Bob（id = 2）和Cam（id = 3）的老师。我想返回用户一行老师和他的学生。例如，如果我正在寻找Al（id = 1），我想返回Al及其学生（Bob和Cam）的用户信息。我以为是

SELECT * FROM users 
  LEFT JOIN classes ON users.id=classes.teacher_id 
  WHERE id=1 OR teacher_id=1

但是我知道这一定是错误的，因为“ teacher_id = 1”无法从classes.student_id获取值。它应该是“ ... WHERE id = {来自Teacher_id = 1的班级的学生ID”。你如何做到的？

Answer 1

您可以使用类似的查询来显示按老师姓名搜索的学生。

SELECT u.* FROM users u
where u.id IN (
    SELECT t.student_id 
    FROM classes t 
    LEFT JOIN users tu ON t.`teacher_id` = tu.id
    WHERE tu.name = 'Al'
    );

样品

MariaDB [bernd]> SELECT * from users;
+----+------+------+
| id | name | age  |
+----+------+------+
|  1 | Al   |   30 |
|  2 | Bob  |    5 |
|  3 | Cam  |    6 |
|  4 | Dan  |    7 |
+----+------+------+
4 rows in set (0.00 sec)

MariaDB [bernd]> SELECT * FROM classes;
+----+------------+------------+
| id | teacher_id | student_id |
+----+------------+------------+
|  1 |          1 |          2 |
|  2 |          1 |          3 |
+----+------------+------------+
2 rows in set (0.00 sec)

MariaDB [bernd]> SELECT u.* FROM users u
    -> where u.id IN (
    ->     SELECT t.student_id
    ->     FROM classes t
    ->     LEFT JOIN users tu ON t.`teacher_id` = tu.id
    ->     WHERE tu.name = 'Al'
    ->     );
+----+------+------+
| id | name | age  |
+----+------+------+
|  2 | Bob  |    5 |
|  3 | Cam  |    6 |
+----+------+------+
2 rows in set (0.00 sec)

MariaDB [bernd]>

Answer 2

尝试此查询

SELECT * FROM classes 
LEFT JOIN user ON classes.teacher_id= users.id
WHERE classes.teacher_id=1 or user.id = 1

Answer 3

您可以将classes加入到users的2个副本中：

select u1.name teacher, u2.name student
from classes c
inner join users u1 on u1.id = c.teacher_id
inner join users u2 on u2.id = c.student_id

请参见demo。
结果：

| teacher | student |
| ------- | ------- |
| Al      | Bob     |
| Al      | Cam     |

或者如果您想用逗号分隔每个老师的所有学生的名单：

select u1.name teacher, group_concat(u2.name) students
from classes c
inner join users u1 on u1.id = c.teacher_id
inner join users u2 on u2.id = c.student_id
group by teacher

请参见demo。
结果：

| teacher | students |
| ------- | -------- |
| Al      | Bob,Cam  |

选择与另一个表中的字段相关的条目

3 个答案: