Primary Key Definition

Definition: The primary key of a relational table uniquely identifies each record in the table. It can either be a normal attribute that is guaranteed to be unique (such as Social Security Number in a table with no more than one record per person) or it can be generated by the DBMS (such as a globally unique identifier, or GUID, in Microsoft SQL Server). Primary keys may consist of a single attribute or multiple attributes in combination. Imagine we have a student records database that contains three tables. The first table, STUDENTS, contains a record for each student at the university. The second table, CLASSES, contains a record for each class session offered. The third table, ENROLLMENT, contains student enrollment records (e.g. each record represents a single student enrolling in a single course). There would be multiple records for each student (representing all the classes that student is enrolled in) and multiple records for each class session (representing all the students enrolled in that class). A student's unique student ID number would be a good choice for a primary key in the STUDENTS table. The student's first and last name would not be a good choice, as there is always the chance that more than one student might have the same name.