MySQL Update a field value with subquery with mult

Question

I have two tables "bank" and "bonds". Each user has a bank record but can have 0, 1, or more bonds records.

I want to write a script that updates the field "cash" in the "bank" table with the interests of the multiple bonds a user might hold. The interest is calculated by issuePrice * coupon fields of the "bonds" table. But since a user might hold multiple bonds, it should do this for each bond.

At the moment, I tried something like this:

$MySQL->db_Query("UPDATE bonds bo, bank ba SET 
        ba.cash = ROUND(ba.cash + (bo.issuePrice * bo.coupon), 2), 
        ba.earned = ROUND(ba.earned + (bo.issuePrice * bo.coupon), 2) 
    WHERE LOWER(ba.user) = LOWER(bo.holder) AND 
        LOWER(bo.holder) <> LOWER('Bank');");

But it doesn't give the expected outcome. I tried it with a user with 2 bonds, if both bonds should give 500 interest each, so a total of 1000, it only adds 500 like there is only 1 bond. If I set one bonds as 500 interest and the other one with an calculated interest of 1000, it suddenly adds 475.

Answer 1

It's probably worthwhile to ensure that your UPDATE statement is trying to update each user's row exactly once. A subquery is the best way to do this, most efficiently implemented as a joined table:

UPDATE bank 
JOIN (SELECT LOWER(bonds.holder) as user,
    SUM(bonds.issuePrice * bonds.coupon) as total
    FROM bonds
    WHERE LOWER(bonds.holder) != 'bank'
    GROUP BY user
) as increments ON increments.user = LOWER(bank.user)
SET bank.cash = ROUND(bank.cash + increments.total, 2),
    bank.earned = ROUND(bank.earned + increments.total, 2)

(For more optimization, the LOWER and ROUND calls should probably be eliminated, but that's another discussion.)

Answer 2

The most straighforward way is to use sub-selects and update the fields individually...

UPDATE bank ba1
SET ba1.cash = ba1.cash + (ROUND(SELECT SUM(bo.issuePrice * bo.coupon)
               FROM bank ba2 JOIN bonds bo ON bo.user = ba2.user
               WHERE ba2.user = ba1.user), 2)
...