Question

I’m trying to write a SELECT query that joins multiple one-to-many tables. A table named disbursements has both a LEFT JOIN to a table named contacts (because a contact can have many disbursements) and a LEFT JOIN to a table named pledges (because a pledge (promise of payment) can have multiple disbursements (actual payments).

My question is how can I pull records that list contacts, their pledges, and the pledge balance (the sum of disbursements towards a pledge subtracted from the pledge)?

Here is what I got so far:

SELECT *, (p.pl_amount - SUM(disb_amount)) as balance FROM disbursements d
LEFT JOIN contacts c on c.c_no = d.c_no
LEFT JOIN  pledges p on d.pl_no = p.pl_no 
GROUP BY d.disb_no
HAVING balance > 500
ORDER BY c.c_last

Thank you in advance

Answer 1

What you’ll probably want to do is put the contacts table in the FROM clause instead of in a LEFT JOIN since you still want to retrieve contacts even when they don’t have any disbursements/pledges.

Also since you seem to only want contacts that have a pledge balance > 500, a LEFT JOIN isn’t necessary unless you also want to retrieve contacts that DON’T have any pledges OR have a balance > 500. I used a regular (inner) JOIN instead:

Try this:

SELECT    c.*,
          p.*,
          COALESCE(p.pl_amount - pd.disb_sum, 0) AS pledge_balance
FROM      contacts c
LEFT JOIN disbursements d ON c.c_no = d.c_no
LEFT JOIN pledges p ON d.pl_no = p.pl_no
LEFT JOIN (
          SELECT   pl_no, SUM(disb_amount) AS disb_sum
          FROM     disbursements
          GROUP BY pl_no
          ) pd ON p.pl_no = pd.pl_no
WHERE     p.pl_no IS NULL OR
          p.pl_amount - pd.disb_sum > 500
ORDER BY  c.c_last