SQL IsNumeric not working

Question

The reserve column is a varchar, to perform sums on it I want to cast it to a deciaml. But the SQL below gives me an error

select
cast(Reserve as decimal)
fr         


        

Answer 1

Gosh, nobody seems to have explained this correctly. SQL is a descriptive language. It does not specify the order of operations.

The problem that you are (well, were) having is that the where does not do the filtering before the conversion takes place. Order of operations, though, is guaranteed for a case statement. So, the following will work:

select cast(case when isnumeric(Reserve) = 1 then Reserve end as decimal)
from MyReserves
where isnumeric(Reserve ) = 1 and MyReserves is not null

The issue has nothing to do with the particular numeric format you are converting to or with the isnumeric() function. It is simply that the ordering of operations is not guaranteed.