What is the function you would use to transfer a string into a decimal?

    In this case no specific function is required if the size of the string and decimal is same. Just use decimal cast with the size in the transform function and will suffice. For example, if the source field is defined as string(8) and the destination as decimal(8) then (say the field name is field1).

out.field :: (decimal(8)) in.field

If the destination field size is lesser than the input then use of string_substring function can be used likie the following.
say destination field is decimal(5).

out.field :: (decimal(5))string_lrtrim(string_substring(in.field,1,5)) /* string_lrtrim used to trim leading and trailing spaces */