to give only string values to a particular field but not numbers or any special characters in oracle?

Answer 1

Put a check constraint on the column similar to:

...
myCol varchar2(20)
constraint myCol_badvalue check (
translate(upper(myCol),
'_ABCDEFGHIJKLMNOPQRSTUVWXYZ','_') IS NULL )

The above will check that the value is A-Z (either case) and if not, return an oracle error because it failed the check.