how can i compare the character fields?

I have a requirement to compare the character fields .

I have to compare the 2 character fields of a table CDPOS

As follows .

If CDPOS-VALUE-OLD GT CDPOS-VALUE-NEW

Populate REDATED = ‘YES’.

ELSE

Populate REDATED = ‘NO’.

My doubt here is how can we compare the character fields.?

When I do Extended Program Check: I am getting error like this ?

Greater than/less than comparisons with character type operands may not be portable

Please give me idea .

Thanks ,

Suresh Kumar.