>HI,
>Is unique of record a business rule? Ex. Some customers may say, same item only can appear once in a invoice. Qty is the place to order more than one. However, the orther customers may say, same item can appear more than one time in a invoice, unless there are different price.
>
>So, should I place the duplication checking un business rules or UI?
>
>Thank you

John,

One of the differences between a file based table and a backend based table is that the backend needs a unique indicator for each record. A file based table uses the record pointer.

Generally, the solution should be in the business rules. Here you can use two unique values. The candidate key, which allows users to retrieve values based on, say, an abbreviation, and a surrogate key, which is generally an identity column which is used to link the various tables.

This technique allows for the change of the candidate key without affecting the relations within the tables.