sql server - 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)
from MyReserves

Error converting data type varchar to numeric.

I added the isnumeric and not null to try and avoid this error but it still persists, any ideas why?

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

Answer 1

See here: CAST and IsNumeric

Try this:

WHERE IsNumeric(Reserve + '.0e0') = 1 AND reserve IS NOT NULL

UPDATE

Default of decimal is (18,0), so

declare @i nvarchar(100)='12121212121211212122121'--length is>18 
SELECT ISNUMERIC(@i) --gives 1
SELECT CAST(@i as decimal)--throws an error