Multiple many-to-many relationships in SQL

Question

How can I query multiple many-to-many relationships in the same result set?

I have two tables that I typically always LEFT JOIN for a standard result set:

tblPROJECTS-

id  |  jobnumber  |  jobname ...
--------------------------------------------------
1   |  1000       |  Project X
2   |  2000       |  Project Y
3   |  3000       |  Project Z

tblTASKS-

id  |  tasknumber |  jobnumber  |  taskname ...
--------------------------------------------------
1   |  10         |  1000       |  Project X: Task 1
2   |  20         |  1000       |  Project X: Task 2
3   |  30         |  2000       |  Project Y: Task 1

Tasknumber is a GUID, independent of jobnumber, but will never be related to more than one job.

I LEFT JOIN tblTASKS on jobnumber, since not all projects will have tasks (yet)

But then I also have an owners table that defines 1-n users who own either the job as a whole or the individual tasks (or both). Each user can own multiple jobs and/or tasks. The original design of the DB spec'd that a single table be used.

tblOWNERS-

id  |  ownertype  |  ownerid  |  jobnumber  |  tasknumber ...
----------------------------------------------------------------
1   |  1          |  2        |  1000       |
2   |  1          |  4        |  1000       |
3   |  2          |  2        |             |  10

An ownertype of 1 indicates the user owns the overall job. An ownertype of 2 indicates the user owns the task within the job.

I have two queries that I'm trying to construct:

1) Return the job with all associates job owners, joined with all tasks for that job with all associated task owners.

jobnumber |  jobowners | tasknumber | taskowners ...
1000      |  2,4,...   | 10         | 2
2000      |            | 20         | 4,6,8...
3000      |  4,5,6...  | 30         | 

2) Given an owner ID, return all the jobs and/or tasks they are associated with.

It's the multiple many-to-many from/to the same tables that has me stumped. Can I accomplish this? If so, am I looking for some sort of UNION or INTERSECT (what do I look up to learn)? Or, if not, what's the better schema for relationships like this that would allow for it?

TIA!

Answer 1

Generally, you need to place the foreign key in the many end of an ERD, so in this case you might have a field called 'ownerid' in the table 'tblPROJECTS', as well as having 'ownerid' in tblTASKS. Assuming then that all tasks have a job ID and an owner, and all projects also have an owner, you can use INNER JOINs:

SELECT P.jobnumber,T.tasknumber,O1.id AS taskowner,O2.id AS jobowner
FROM tblTASKS T
INNER JOIN tblPROJECTS P ON P.jobnumber=T.jobnumber
INNER JOIN tblOWNERS O1 ON O1.id=T.ownerid
INNER JOIN tblOWNERS O2 ON O2.id=P.ownerid
WHERE O1.id=1

This will not concatenate the jobowners and task owners as you have described, but will return a row for each, which you can then concatenate whilst processing the resultset.

Then just replace the WHERE clause as necessary to get the list of tasks for a given Job number...

WHERE P.jobnumber=1000