Function that will select value from SQL table and do a sum based on another column value

Question

I need a query to sum up the hours in a table. Which hours need to be selected depends on the Start Date column. If the start date is in the past, then it should take Remaining Hours , if it's in the future it should take Budgeted Hours .

Start Date	Budgeted Hours	Remaining Hours
Jan 1, 2022	15	3
Feb 1, 2022	12	0
Mar 1, 2022	14	6
Apr 1, 2022	15	13

In the example above the summed up hours should be 24 (3 + 0 + 6 + 15).

Answer 1

You don't need a function for this, just a CASE expression:

DECLARE @now datetime = GETDATE();

SELECT SUM
(
  CASE WHEN [Start Date] < @now 
  THEN [Remaining Hours]
  ELSE [Budgeted Hours] 
  END
)
FROM dbo.tablename;

Answer 2

We can use the function case

 create table ops( startDate date, Budgeted int, Remaining int);

 insert into ops values ('2022-01-01',15,3), ('2022-02-01',12,0), ('2022-03-01',14,6), ('2022-04-01',15,13), ('2022-05-01',10,18);

 select startDate, Budgeted, Remaining, case when startDate < getDate() then Remaining else Budgeted end Hours from ops;

 startDate | Budgeted | Remaining | Hours:--------- | -------: |  --------: |  ----: 2022-01-01 |  15 |  3 |  3 2022-02-01 |  12 |  0 |  0 2022-03-01 |  14 |  6 |  6 2022-04-01 |  15 |  13 |  15 2022-05-01 |  10 |  18 |  10 

db<>fiddle here