Substraction in MYSQL with some condition - mysql

Question

I want to do SUM and SUBTRACTION upon certain condition within mysql query
i have try like this,
asuming the $balance= 655.000.000 then total amount inside my_table = IDR 80.000.000 and USD 500.000
$currency_id = 1;
$balance = 655.000.000;
DB::table('my_table as a')
->join('master_currencies as b', 'a.master_currency_id', 'b.id')
->where('a.id', $id)
->selectRaw("SUM(
CASE
WHEN $currency_id = a.master_currency_id THEN $balance - a.amount
ELSE
a.amount
END) AS net_value, b.symbol, b.id as master_currency_id"
)
->groupBy('b.id')
->get();
The code above should give the result
[
{"net_value":"575.000.000","symbol":"IDR","master_currency_id":1},
{"net_value":"500.000","symbol":"USD","master_currency_id":2}
]
but the result i got is
[
{"net_value":"1.885.000.000","symbol":"IDR","master_currency_id":1},
{"net_value":"500.000","symbol":"USD","master_currency_id":2}
]
Note:
Please ignore dots between numbers, in my database i'm not using these dots. it is for inquiry purposes only

Answer 1

If you have more than one row of IDR then it might be a problem with order of operations.
For example say you have to values (100.000.000) and (50.000.000) then for IDR SUM(655.000.000-100.000.000, 655.000.000-50.000.000)=1.160.000.000 and is not the same as 655.000.000-SUM(100.000.000,50.000.000)=505.000.000 the problem being that each row in the group that you aggregate gets a clean $balance.
You could potentially solve this by doing:
$currency_id = 1;
$balance = 655.000.000;
DB::table('my_table as a')
->join('master_currencies as b', 'a.master_currency_id', 'b.id')
->where('a.id', $id)
->selectRaw("
(CASE
WHEN a.id=$currency_id $balance-SUM(a.amount)
ELSE
SUM(a.amount)
END) AS net_value, b.symbol, b.id as master_currency_id")
->groupBy('b.id')
->get();