Question

I think my problem is solved with a ‘recursive query’, but since MySQL doesn’t support recursive queries I was trying to use the adjacent list model. This should not be a problem since I know how deep I want to go.

Here’s an example of what I need to do:
Table Classes:

dept    classNum    prereqDept  prereqClassNum
BIO     465         BIO         335
EE      405         EE          325
EE      325         EE          120
BIO     465         EE          120
BIO     335         BIO         225
BIO     225         CHEM        110
BIO     225         BIO         105

What I need is all the classes of a certain level (let’s say 400) with all their prerequisites up to 3 levels deep.
So I’d get something like

dept    classNum    prereqDept  prereqClassNum
BIO     465         BIO         335
BIO     465         BIO         225
BIO     465         CHEM        110
BIO     465         BIO         105
EE      405         EE          325
EE      405         EE          120
....

I know I need to use 3-LEFT JOINs if I want to go 3 levels deep, but I can’t figure out how to set up these joins to get what I need.
Any ideas?
I’ll appreacite your help!

P.S. I can’t change the table structure at all.

Answer 1

Hmm. Try this:

SELECT * FROM
(
    (
        SELECT t1.dept, t1.classNum, t1.prereqDept, t1.prereqClassNum
        FROM class AS t1
        WHERE t1.classNum >= 400
    )
    UNION
    (
        SELECT t1.dept, t1.classNum, t2.prereqDept, t2.prereqClassNum
        FROM class AS t1
        JOIN class AS t2 ON (t1.prereqDept = t2.dept AND t1.prereqClassNum = t2.classNum)
        WHERE t1.classNum >= 400
    )
    UNION
    (
        SELECT t1.dept, t1.classNum, t3.prereqDept, t3.prereqClassNum
        FROM class AS t1
        JOIN class AS t2 ON (t1.prereqDept = t2.dept AND t1.prereqClassNum = t2.classNum)
        JOIN class AS t3 ON (t2.prereqDept = t3.dept AND t2.prereqClassNum = t3.classNum)
        WHERE t1.classNum >= 400
    )
) AS t4
ORDER BY dept, classNum, prereqDept, prereqClassNum