To enforce the LitCrit business rule of limiting all borrowers to one critique per title, a UNIQUE constraint (a constraint that enforces uniqueness) is created in the Critique table in the FmLib database. In the following illustration, the system generated the name "IX_critique" for this constraint; you can rename it by entering a new name in the Index name text box. (You can later use the constraint's name to modify or delete it.) The constraint operates on the concatenated borrower# and bib# columns, specifying that each combination of a borrower# and a bib# can only occur once.
As an alternative to a UNIQUE constraint on the concatenated borrower# and bib# columns, the Critique table could use borrower# and bib# as a compound primary key. However, using the critique# as the primary key and enforcing the business rule with a UNIQUE constraint has the advantage of future flexibility. Should the business rule change to allow additional critiques of a title by a borrower, it is straightforward to delete the UNIQUE constraint; no modification to the Critique table would be necessary. It would be more complex and time-consuming to add a new primary key and remove the old concatenated primary key, borrower# and bib#, from the Critique table.