Select entries in relation to fields in another table [closed] - mysql

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I have the following tables, Table1 shows all users, and Table2 shows a teacher-to-student relationship:
Table1 (users):
ID name age
-------------
1 Al 30
2 Bob 5
3 Cam 6
4 Dan 7
Table2 (classes):
teacher_id student_id
----------------------
1 2
1 3
Essentially, this shows Al (id=1) is the teacher of Bob (id=2) and also Cam (id=3). I want to return users rows of a teacher and his students. For example, if I'm looking for Al (id=1), I want to return users info of Al and his students (Bob and Cam). What I thought was
SELECT * FROM users
LEFT JOIN classes ON users.id=classes.teacher_id
WHERE id=1 OR teacher_id=1
But I know it must be wrong because "teacher_id=1" doesn't get values from classes.student_id. It should be "... WHERE id={the student_id's from classes where teacher_id=1}". How do you do this?

Answer 1

You can use a Query like this to show the students search by teachers name.
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'
);
Sample
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

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

Answer 3

You can join classes to 2 copies of users:
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
See the demo.
Results:
| teacher | student |
| ------- | ------- |
| Al | Bob |
| Al | Cam |
Or if you want a comma separated list of all the students of each teacher:
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
See the demo.
Results:
| teacher | students |
| ------- | -------- |
| Al | Bob,Cam |