Curious as to know why this doesn't work MySQL - mysql

Question

What I'm trying to do is get every teacher name involved with the class_id provided. And it does nothing, very strange.The tables the query is connected to:
+----+------------+----------+
| id | teacher_id | class_id |
+----+------------+----------+
| 1 | 1 | 2 |
| 6 | 1 | 4 |
| 10 | 2 | 3 |
| 5 | 2 | 5 |
| 4 | 3 | 1 |
| 7 | 3 | 3 |
| 9 | 4 | 2 |
| 2 | 4 | 4 |
| 3 | 5 | 1 |
| 8 | 5 | 5 |
+----+------------+----------+
10 rows in set (0,00 sec)
mysql> select * from classes;
+----------+------------+------------------------+
| id_class | class_name | responsible_teacher_id |
+----------+------------+------------------------+
| 1 | Geography | 2 |
| 2 | Reading | 1 |
| 3 | Science | 3 |
| 4 | History | 5 |
| 5 | Arithmetic | 4 |
+----------+------------+------------------------+
5 rows in set (0,00 sec)
mysql> select * from teachers;
+----+------------+-------------+-----------+------------+------------+-------------+----------------------------------+
| id | first_name | middle_name | last_name | birthdate | subject_id | current_age | md5 |
+----+------------+-------------+-----------+------------+------------+-------------+----------------------------------+
| 1 | Oliver | Weed | Saxon | 1983-01-03 | 2 | 38 | 4bf51ce48af83dba7c8390a7222db991 |
| 2 | Dexter | Tristan | Morgan | 1980-12-11 | 5 | 40 | be96d6213828fd09538d8b1357907d54 |
| 3 | Deborah | Ocean | Morgan | 1985-06-10 | 2 | 35 | ba6869ece0a74ca9987b83b4f840405b |
| 4 | George | Kai | Smith | 1965-01-03 | 6 | 56 | ccec54863dbb6b63e55d495341fd9a63 |
| 5 | Chi | Kai | Chian | 1979-10-03 | 3 | 41 | 2291ce528bf05eff905af1eecd9bc53b |
+----+------------+-------------+-----------+------------+------------+-------------+----------------------------------+
5 rows in set (0,00 sec)
And the query:
select first_name
, middle_name
, last_name
FROM teachers_seth t
JOIN classes_sethputz c
ON c.responsible_teacher_id = t.id
JOIN teachers_classes_sethputz t_c
ON t_c.class_id = c.id_class
AND t_c.teacher_id = t.id
WHERE c.id_class = 1;
And what the query returned each time I ran it with a different class_id:
c.id_class = 1;
Empty set (0,00 sec)
c.id_class = 2;
+------------+-------------+-----------+
| first_name | middle_name | last_name |
+------------+-------------+-----------+
| Oliver | Weed | Saxon |
+------------+-------------+-----------+
1 row in set (0,00 sec)
c.id_class = 3;
+------------+-------------+-----------+
| first_name | middle_name | last_name |
+------------+-------------+-----------+
| Deborah | Ocean | Morgan |
+------------+-------------+-----------+
1 row in set (0,00 sec)
c.id_class = 4;
Empty set (0,00 sec)
c.id_class = '5';
Empty set (0,00 sec)
Thanks. This is really strange for me.

Answer 1

You have too much joining going on for your question, you have 3 tables, teachers, classes and a matrix that links them many-many.
You are joining the classes table directly to the teachers table with the responsible_teacher_id column, this should not be there if you want the teachers involved with the classes. What you are doing is saying if the teachers are the responsible teacher AND the teacher is involved in the class and that is rarely the case so you get very few hits.
So to solve it, start with the class table (classes_sethputz), as that is what you appear to be doing and then add in the matrix table (teachers_classes_sethputz) by joining it then add in the teachers table (teachers_seth) by joining it to the matrix table (teachers_classes_sethputz).
SELECT first_name
, middle_name
, last_name
FROM classes_sethputz c
JOIN teachers_classes_sethputz t_c
ON t_c.class_id = c.id_class
JOIN teachers_seth t
ON t.id = t_c.teacher_id
WHERE c.id_class = 5;