problem in defining user defined tables

Hi all

I have one doubt while creating user defined table.

When i create a user defined table: say XYZ, two fields come by default namely "code" and "name". As per logic, the 2 should act as composite key, but it gives me an error while i enter values in these fields.

Example

code name

1 a

2 b

3 c

4 "a" when i enter "a" here, it is not allowing to

add "a" and gives an error "cannot insert key in row in object '@XYZ' with unique index.

Pls help