Determining Unique Attributes (Designing Databases)
Go Up to Designing Databases
One of the tasks of database design is to provide a way to uniquely identify each occurrence or instance of an entity so that the system can retrieve any single row in a table. The values specified in the primary key of the table distinguish the rows from each other. A
PRIMARY KEY or
UNIQUE constraint ensures that values entered into the column or set of columns are unique in each row. If you try to insert a value in a
PRIMARY KEY or
UNIQUE column that already exists in another row of the same column, InterBase prevents the operation and returns an error.
For example, in the
EMP_NO is a unique attribute that can be used to identify each employee in the database, so it is the primary key. When you choose a value as a primary key, determine whether it is inherently unique. For example, no two social security numbers or driver’s license numbers are ever the same. Conversely, you should not choose a name column as a unique identifier due to the probability of duplicate values. If no single column has this property of being inherently unique, then define the primary key as a composite of two or more columns which, when taken together, are unique.
A unique key is different from a primary key in that a unique key is not the primary identifier for the row, and is not typically referenced by a foreign key in another table. The main purpose of a unique key is to force a unique value to be entered into the column. You can have only one primary key defined for a table, but any number of unique keys.