Question

Let’s say I have 3 mysql tables – teacher, subject, and class, each table having only two fields id and name

If every teacher can teach every subject in every class, what is a good way to insert this information using SQL into a fourth table to hold that data, if my fourth table has three fields for teacher_id, subject_id, and class_id

Just to make it clear by an example, if I had only two teachers (teacher.id 1 and 2), two subjects (subject.id 1 and 2) and two classes (class.id 1 and 2), data in the fourth table would look something like below.

teacher_id  subject_id  class_id
    1           1           1
    1           1           2
    1           2           1
    1           2           2
    2           1           1
    2           1           2
    2           2           1
    2           2           2

Answer 1

Seems like you are looking at a cross join or cartesian product.

http://en.wikipedia.org/wiki/Join_%28SQL%29#Cross_join

In your case

SELECT teacher_id, subject_id, class_id 
    FROM teacher JOIN subject JOIN class 
    ORDER BY teacher_id, subject_id, class_id

With large sets of data this can take a while to return.