Update value of row in mysql with conditions? - php

Question

I want to achieve something like this using php and mysql
if the customer has an account with rewards and wants to spend rewards, how do i update the table so that it will going to subtract the spent reward to the table.
Definitely it will going to get the sum of reward by customer_id then subtract the spent reward. IF the first row(reward) is less than the spent value, it will going to subtract all then go to next row get the difference from previous result until the value of spent is equal to 0.
sample:
spent = 60
id_customer = 2
I have a table like this
id | id_customer | reward
1 | 2 | 50
2 | 2 | 20
3 | 3 | 100
4 | 4 | 5
the result should be something like this:
1st row: 50(value of first row) - 60 = 0 (with remaining 10)
2nd row: 20(value of 2nd row) - 10 (remaining points from first row) = 0
id | id_customer | reward
1 | 2 | 0
2 | 2 | 10
3 | 3 | 100
4 | 4 | 5
Hope that makes sense. Thanks

Answer 1

This is the logic of my solution (of course, maybe more than this one and a better ones):
Get the set of rows for that customer (id_customer = 2) and loop through the rows returned.
In each iteration, compare the value of field reward against the amount you like to subtract (60).
If the actual value is >= 60, update that row and exit. If not, update it with 0, update the remain value (60 - row value) and go to the next item in the iteration doing the same action.

Answer 2

In MySQL, I think the best way to do this is with variables. This should work:
declare #spent := 60;
update tablelikethis
set reward = (case when #spent = 0 then reward
when #spent >= reward
then (case when (#tmp := #spent) is null then NULL
when (#spent := #spent - reward) is null then NULL
else 0
end)
else (case when (#tmp := #spent) is null then NULL
when (#spent := 0) is null then NULL
else reward - #tmp
end)
end)
where id_customer = 2
order by id;
MySQL makes this a little hard to do in a single update, because you cannot use order by with a join. The variable version just has to deal with logic on whether the amount remaining for the reward is bigger or less than the amount remaining being spent.

Answer 3

I'd structure your database in a different way. You could create a column called "reward_points" in the customer table, and have a separate reward table. The structure is:
REWARD_TABLE
----------------------------------
reward_id | customer_id | reward
----------------------------------
1 | 2 | 50
2 | 2 | 20
3 | 3 | 100
4 | 4 | 5
CUSTOMER_TABLE
-------------------------------------------------
customer_id | name | reward_points
-------------------------------------------------
1 | Eddard Stark | 0
2 | Jaime Lannister | 70
3 | Joffrey Baratheon | 100
4 | Theon Greyjoy | 5
Then you could just update the CUSTOMER_TABLE with the new value. You could keep the REWARD_TABLE as a 'reward history'. Even better... upon purchase, you could add a negative transaction to the REWARD_TABLE, so when you would do a SELECT SUM(reward) asRewardFROM reward_table WHERE customer_id = 2 GROUP BY customer_id, it would count all the negative transactions as well, resulting in something, which is close to your concept.