MySQL: How do I select two fields from table, and total of amount columns in two related tables?

Question

I have three tables:

Project PurchaseA PurchaseB

Schema of Project:

id | user_id | title | description | create_date

Schema of PurchaseA:

id | project_id | amount | create_date

Schema of PurchaseB:

id | project_id | amount | create_date

Project has a bunch of unique rows detailing different projects. Each of these projects has many rows in both PurchaseA and PurchaseB . What I want to do is select all of the rows from Project with the fields id and title , but also the SUM of both the total of the related records in PurchaseA and the total of the related records in PurchaseB .

I can get it working sort of independently of each other, but when I have two joins, the numbers end up being massively inflated.

Desired Result

21732 Test Project A          84274.48
21747 Test Project B          18237.47
34714 Test Project C         183747.18

Where the column with the FLOATS is the sum of the related rows from PurchaseA and PurchaseB .

Edit

Here is my current SQL:

select 
    projects.id,
    projects.title,
    COALESCE(SUM(PurchaseA.amount), 0) as pledgetotal,
    COALESCE(SUM(PurchaseB.amount), 0) as purchasetotal
from
    projects
        left join
    PurchaseA ON (PurchaseA.project_id = projects.id)
        left join
    PurchaseB ON (PurchaseB.project_id = projects.id)
        left join

WHERE
    projects.title IS NOT NULL

    projects.end_date < NOW()
group by projects.id

Test Data

Projects:

id          title           description         create_date

623         Test Project A  This is a test A    2013-01-01
624         Test Project B  This is a test B    2013-01-02


PurchaseA

id          project_id      amount              create_date
1           623             100                 2013-01-02
2           623             125                 2013-01-02
3           624             200                 2013-01-03
4           623             150                 2013-01-03
5           624             50                  2013-01-04

PurchaseB

id          project_id      amount              create_date
1           623             110                 2013-01-02
2           624             105                 2013-01-02
3           623             215                 2013-01-03
4           623             100                 2013-01-03
5           624             150                 2013-01-04

So as you can see, each project has multiple rows in each related table. I want to get the sums from each table, then add those together.

Answer 1

I believe part of your problem is from the one-to-many relationship that exists between the projects and the purchase. tables. My suggestion would be to move the sum() to subqueries:

select 
    p.id,
    p.title,
    a.pledgetotal,
    b.purchasetotal
from projects p
left join
(
  select project_id, sum(amount) pledgetotal
  from PurchaseA
  group by project_id
) a
  ON (a.project_id = p.id)
left join
( 
  select project_id, sum(amount) purchasetotal
  from PurchaseB
  group by project_id
) b
  ON (B.project_id = p.id)
WHERE p.title IS NOT NULL;

See SQL Fiddle with Demo

Answer 2

You can try this (not tested):

SELECT a.id, a.title, (pa.amount+pb.amount)  AS TotalPurchase FROM Project a
    LEFT JOIN ( SELECT project_id, SUM(amount) AS amount FROM PurchaseA GROUP BY project_id ) pa
    ON pa.project_id = a.id
    LEFT JOIN ( SELECT project_id, SUM(amount) AS amount FROM PurchaseB GROUP BY project_id ) pb
    ON pb.project_id = a.id