mysql join for specific rows only - php

Question

i've three tables
comments table
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id | commented_by | comment | commented_by
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1 | asdf156df5s4fd | student comment | student
2 | klsdfjklsdfno | student comment | student
3 | jafkadfjlkdnvwa | teacher comment | teacher
4 | adsjflandsfinndfs | student comment | student
5 | dsf5614dsf8wef4 | teacher comment | teacher
---------------------------------------------------------------------------
teacher table
id | teacher_id | teacher_name | teaches
---------------------------------------------------------------------------
1 | jafkadfjlkdnvwa | abc cde | maths
2 | dsf5614dsf8wef4 | egc kjl | science
.
.
.
---------------------------------------------------------------------------
student table
id | student_id | student_name |
---------------------------------------------------------------------------
1 | asdf156df5s4fd | student_1 |
2 | klsdfjklsdfno | student_2 |
3 | adsjflandsfinndfs | student_3 |
.
.
---------------------------------------------------------------------------
now, what we wish is mysql join which could return something like this
id | comment_id | comment_by | name | teaches
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1 | asdf156df5s4fd | student | student_1 | null
2 | klsdfjklsdfno | student | student_2 | null
3 | jafkadfjlkdnvwa | teacher | abc cde | maths
4 | adsjflandsfinndfs | student | student_3 | null
5 | dsf5614dsf8wef4 | teacher | egc kjl | science
---------------------------------------------------------------------------
unable to find the exact query with which we can achieve this with codeigniter active records

Answer 1

You need to join student and teacher table:
select id, comment_id as comment_by , commented_by,
( case when commented_by = 'teacher' THEN teacher_name else student_name ) AS name, teaches
from comments c left join teacher t ON t.teacher_id = c.commented_by left
join student s ON s.student_id = c.commented_by

Answer 2

It's real dirty table scheme but this is the query you want:
SELECT
c.id,
commented_by as comment_id,
commented_by,
IF(commented_by = 'student', student_name, teacher_name) as name,
teaches
FROM comments c
LEFT JOIN teacher t
ON t.teacher_id = c.commented_by
AND c.commented_by = 'teacher'
LEFT JOIN student s
ON s.student_id = c.commented_by
AND c.commented_by = 'student'
A better table scheme:
CREATE TABLE comments (
comment_id INT NOT NULL AUTO_INCREMENT,
student_id INT,
teacher_id INT,
PRIMARY KEY (comment_id)
);
CREATE TABLE teachers (
teacher_id INT NOT NULL AUTO_INCREMENT,
teacher_name VARCHAR(200) NOT NULL,
teacher_subject VARCHAR(50),
PRIMARY KEY (teacher_id)
);
CREATE TABLE students (
student_id INT NOT NULL AUTO_INCREMENT,
student_name VARCHAR(200) NOT NULL,
PRIMARY KEY (student_id)
);
SELECT
comments.*,
IF(student_id IS NOT NULL, student_name, teacher_name) as name,
teacher_subject
FROM comments
LEFT JOIN teachers USING(teacher_id)
LEFT JOIN students USING(student_id)