Question

    select top 10 *, case     
    when datediff(day,DateOfSale, getDate())  > 5 then '5'
        when datediff(day,DateOfSale, getDate())  > 10 then '10'
... (more datediff clauses)
...
...
        else '20'
        end as jack
        from Foo

Is SQL Server smart enough to evaluate the datediff function call once within the case statement and then use that value for every when clause? Or is the function is getting called ‘n’ times, where ‘n’ is the amount of when clauses?

Answer 1

It’s hard to see how SQL Server could evaluate the call once. The call has a column as parameter and so has to be evaluated for every row.

Thus, your condition is better written like:

when DateOfSale < dateadd(day, -5, getdate()) then '5'

In this case the difference is small. Date calculations are cheap.

The classic example where the function call does matter is a where condition on a table with an index on the date column. For example, YourTable with an index on (dt). This query would allow an index to be used:

select * from YourTable where dt < dateadd(day, -5, getdate())

While this query would not:

select * from YourTable where datediff(day, DateOfSale, getDate()) > 5