Table-Level Candidate Key Constraint

If an application requires that the values for a combination of columns be unique, then the candidate key specification is to be done at the table level. For example, in an order_item table the columns order_no and item_no together form a unique key.

Consider the following example:

CREATE TABLE order_item (

     order_no    INTEGER NOT NULL,

     item_no     INTEGER NOT NULL,

     qty         INTEGER,

     price       MONEY

     UNIQUE (order_no, item_no)

) ;

In the above example, the combination of columns order_no and item_no is the unique identifier of the table order_item.