top 1 first_value() based on two criteria - sql-server

Question

I'm still new at mssqlserver, i need to get the first order for the same customer made order in certain period based on the brand name and customer_id I used first_value() as sub query with top 1 however i got only the first value based on the brand regardless the customer id
what i got
Data
Customer_ID
Brand
first_order
2022-01-01
1649104
PH
2020-09-26
2022-01-01
1306498
PH
2020-09-26
2022-01-01
1290371
PH
2020-09-26
2022-01-01
456756
TB
2020-11-09
2022-01-01
1823713
TB
2020-11-09
2022-01-01
2178025
BK
2020-08-12
2022-01-01
216435
BK
2020-08-12
2022-01-01
19031
BK
2020-08-12
2022-01-01
438095
BK
2020-08-12
the code I used
SELECT Data , Brand , Customer_ID ,
(select top 1 first_value(H.Data) over (partition by Customer_ID , Brand order by H.Data ASC)
from ORDER H
where h.Customer_ID = Customer_ID and H.Brand = Brand
) as firts_order
from ORDER
LEFT OUTER JOIN ORDER_SOURCE SRC ON SRC.SRC_ID = ORDR_SOURCE
WHERE SRC_NAME IN ('SR 1' , 'SR 2') and DATE = '2022-01-01'

Answer 1

I've not used that first_value() window function before, but my bet is that you don't need a subquery. Instead:
SELECT Data,
Brand,
Customer_ID,
FIRST_VALUE(Data) OVER (PARTITION BY Customer_ID, Brand ORDER BY Data) as first_order
FROM ORDER
LEFT OUTER JOIN ORDER_SOURCE SRC ON SRC.SRC_ID = ORDR_SOURCE
WHERE SRC_NAME IN ('SR 1' , 'SR 2') AND DATE = '2022-01-01'
Alternatively you could use a correlated subquery INSTEAD. You don't need to combine this with a window function. Although, I wouldn't recommend it since the window function route would be quicker.
SELECT Data , Brand , Customer_ID ,
(
select top 1 H.data
from ORDER H
where H.Customer_ID = O.Customer_ID
and H.Brand = O.Brand
ORDER BY H.Data ASC
) as first_order
from ORDER O
LEFT OUTER JOIN ORDER_SOURCE SRC ON SRC.SRC_ID = ORDR_SOURCE
WHERE SRC_NAME IN ('SR 1' , 'SR 2') and DATE = '2022-01-01'

Answer 2

after working around I have found a more efficient solution for me using min() Instead of the first_value()
SELECT H.Data , H.Brand , H.Customer_ID , O.Data
from ORDER H
LEFT OUTER JOIN ORDER_SOURCE SRC ON SRC.SRC_ID = H.ORDR_SOURCE
LEFT OUTER JOIN (
select min(Data) AS Data, Brand, Customer_ID
from ORDER
LEFT OUTER JOIN ORDER_SOURCE ON SRC_ID = ORDR_SOURCE
WHERE SRC_NAME IN ('SR 1' , 'SR 2')
GROUP by Brand, Customer_ID )
AS O
ON H.Brand = O.Brand AND H.Customer_ID = O.Customer_ID
WHERE SRC_NAME IN ('SR 1' , 'SR 2') and DATE = '2022-01-01'