Question

I know I’m gonna get down votes, but I have to make sure if this is logical or not.

I have three tables A, B, C. B is a table used to make a many-many relationship between A and C. But the thing is that A and C are also related directly in a 1-many relationship

A customer added the following requirement:

Obtain the information from the Table B inner joining with A and C, and in the same query relate A and C in a one-many relationship

Something like:

alt text http://img247.imageshack.us/img247/7371/74492374sa4.png

I tried doing the query but always got 0 rows back. The customer insists that I can accomplish the requirement, but I doubt it. Any comments?

PS. I didn’t have a more descriptive title, any ideas?

UPDATE: Thanks to rcar, In some cases this can be logical, in order to have a history of all the classes a student has taken (supposing the student can only take one class at a time)

UPDATE: There is a table for Contacts, a table with the Information of each Contact, and the Relationship table. To get the information of a Contact I have to make a 1:1 relationship with Information, and each contact can have like and an address book with; this is why the many-many relationship is implemented.

The full idea is to obtain the contact’s name and his address book. Now that I got the customer’s idea… I’m having trouble with the query, basically I am trying to use the query that jdecuyper wrote, but as he warns, I get no data back

Answer 1

I’m supposing that s.id_class indicates the student’s current class, as opposed to classes she has taken in the past.

The solution shown by rcar works, but it repeats the c1.className on every row.

Here’s an alternative that doesn’t repeat information and it uses one fewer join. You can use an expression to compare s.id_class to the current c.id_class matched via the mtm table.

SELECT s.name, c.className, (s.id_class = c.id_class) AS is_current FROM s JOIN many_to_many AS mtm ON (s.id_student = mtm.id_student)   JOIN c ON (c.id_class = mtm.id_class); 

So is_current will be 1 (true) on one row, and 0 (false) on all the other rows. Or you can output something more informative using a CASE construct:

SELECT s.name, c.className,    CASE WHEN s.id_class = c.id_class THEN 'current' ELSE 'past' END AS is_current FROM s JOIN many_to_many AS mtm ON (s.id_student = mtm.id_student)   JOIN c ON (c.id_class = mtm.id_class);