you can write like this--
declare @var1 as char(20)
set @var1="214"
select case when isnumeric(@var1) =1 then cast(@var1 as float) else null end
or you can directly put the value
select case when isnumeric(col1) =1 then cast(col1 as float) else null end from tablename
select case when isnumeric('525') =1 then 525 else null end
for access the query will change as
select iif(isnumeric(col1) ,cdbl(col1),null) from tablename
and the syntex is-
Case when condition then truepart else falsepart end
iif(condition,truepart,falsepart)
--Pankaj