using left join on mutiple tables with conditional statement

Question

I have a problem with MYSQL which can only be solved by using inner join which i am not so good with, i have four tables.

Table 1: classrooms classroomID, name, .......

Table 2: teachers fname, mname, lname, .......

Table 3: subjects subjectID, name, classroom_id, .....

table 4 Teachers_subjects teacher_id and subject_id

i want a query to get me all class names (classrooms.name) and all its subjects name (subjects.name) where subjects.classroom_id = classrooms.classroomID

I also want to get the names of the teachers (teachers.name) where Teachers_subjects.subject_id = subjects.subjectID and Teachers_subjects.teacher_id= teachers.teacherID.

i am currently using this query :

SELECT classrooms.name, subjects.name, 
concat(teachers.fname,' ',teachers.mname,' ',teachers.lname) as Teachers_name  
FROM classrooms, subjects, teachers, teachers_subjects 
WHERE
subjects.classroom_id=classrooms.classroomID 
AND 
teachers_subjects.teacher_id=teachers.teacherID 
AND
teachers_subjects.subject_id=subjects.subjectID

I get what i want but i want to get all classes and their corresponding subjects even if its not on the table teachers_subjects. this is to list all classes and its subjects even if the subject is not assigned to any teacher.

Thanks

Answer 1

You could try something like

SELECT  classrooms.name, 
    subjects.name, 
    concat(teachers.fname,' ',teachers.mname,' ',teachers.lname) as Teachers_name  
FROM    classrooms INNER JOIN
    subjects ON subjects.classroom_id=classrooms.classroomID  LEFT JOIN 
    teachers_subjects ON teachers_subjects.subject_id=subjects.subjectID LEFT JOIN
    teachers ON teachers_subjects.teacher_id=teachers.teacherID 

Have a look at this a article ( Introduction to JOINs – Basic of JOINs ) for a nice graphical introduction.