Primary Keys

An application might require that database table contain one or more columns that can uniquely identify a row. For example, in a supplier table, it is necessary that the column supp_no uniquely identify a row. Every row of the table is uniquely identified by this column value.

A single column or a group of columns that is the principal unique identifier of the table is referred to as the primary key. A table can contain only one primary key constraint.

The following example shows the creation of a primary key column on the table supplier.

CREATE TABLE supplier (

     supp_no     INTEGER NOT NULL PRIMARY KEY,

     name        CHAR (30),

     status      SMALLINT,

     city        CHAR (20)

) ;

During an INSERT operation, if a duplicate value is given for a primary key column, the INSERT operation results in an error.