Left Join on Associative Table - sql

Question

I have three tables
Prospect -- holds prospect information
id
name
projectID
Sample data for Prospect
id | name | projectID
1 | p1 | 1
2 | p2 | 1
3 | p3 | 1
4 | p4 | 2
5 | p5 | 2
6 | p6 | 2
Conjoint -- holds conjoint information
id
title
projectID
Sample data
id | title | projectID
1 | color | 1
2 | size | 1
3 | qual | 1
4 | color | 2
5 | price | 2
6 | weight | 2
There is an associative table that holds the conjoint values for the prospects:
ConjointProspect
id
prospectID
conjointID
value
Sample Data
id | prospectID | conjointID | value
1 | 1 | 1 | 20
2 | 1 | 2 | 30
3 | 1 | 3 | 50
4 | 2 | 1 | 10
5 | 2 | 3 | 40
There are one or more prospects and one or more conjoints in their respective tables. A prospect may or may not have a value for each conjoint.
I'd like to have an SQL statement that will extract all conjoint values for each prospect of a given project, displaying NULL where there is no value for a value that is not present in the ConjointProspect table for a given conjoint and prospect.
Something along the lines of this for projectID = 1
prospectID | conjoint ID | value
1 | 1 | 20
1 | 2 | 30
1 | 3 | 50
2 | 1 | 10
2 | 2 | NULL
2 | 3 | 40
3 | 1 | NULL
3 | 2 | NULL
3 | 3 | NULL
I've tried using an inner join on the prospect and conjoint tables and then a left join on the ConjointProspect, but somewhere I'm getting a cartesian products for prospect/conjoint pairs that don't make any sense (to me)
SELECT p.id, p.name, c.id, c.title, cp.value
FROM prospect p
INNER JOIN conjoint c ON p.projectID = c.projectid
LEFT JOIN conjointProspect cp ON cp.prospectID = p.id
WHERE p.projectID = 2
ORDER BY p.id, c.id
prospectID | conjoint ID | value
1 | 1 | 20
1 | 2 | 30
1 | 3 | 50
1 | 1 | 20
1 | 2 | 30
1 | 3 | 50
1 | 1 | 20
1 | 2 | 30
1 | 3 | 50
2 | 1 | 10
2 | 2 | 40
2 | 1 | 10
2 | 2 | 40
2 | 1 | 10
2 | 2 | 40
3 | 1 | NULL
3 | 2 | NULL
3 | 3 | NULL
Guidance is very much appreciated!!

Answer 1

Then this will work for you... Prejoin a Cartesian against all prospects and elements within that project via a select as your first FROM table. Then, left join to the conjoinprospect. You can obviously change / eliminate certain columns from result, but at least all is there, in the join you want with exact results you are expecting...
SELECT
PJ.*,
CJP.Value
FROM
( SELECT
P.ID ProspectID,
P.Name,
P.ProjectID,
CJ.Title,
CJ.ID ConJointID
FROM
Prospect P,
ConJoint CJ
where
P.ProjectID = 1
AND P.ProjectID = CJ.ProjectID
ORDER BY
1, 4
) PJ
LEFT JOIN conjointProspect cjp
ON PJ.ProspectID = cjp.prospectID
AND PJ.ConjointID = cjp.conjointid
ORDER BY
PJ.ProspectID,
PJ.ConJointID

Answer 2

Your cartesian product is a result of joining by project Id - in your sample data there are 3 prospects with a project id of 1 and 3 conjoints with a project id of 1. Joining based on project id should then result in 9 rows of data, which is what you're getting. It looks like you really need to join via the conjointprospects table as that it what holds the mapping between prospects and conjoint.
What if you try something like:
SELECT p.id, p.name, c.id, c.title, cp.value
FROM prospect p
LEFT JOIN conjointProspect cp ON cp.prospectID = p.id
RIGHT JOIN conjoint c ON cp.conjointID = c.id
WHERE p.projectID = 2
ORDER BY p.id, c.id
Not sure if that will work, but it seems like conjointprospects needs to be at the center of your join in order to correctly map prospects to conjoints.