3.1. Primary Key
The PRIMARY KEY of a table constrains a single column, or set of columns, to a unique and mandatory value—mandatory, meaning that no column in the primary key can ever be null. A table need not have a primary key, but this is the exception; most tables are created with a primary key.
Consider a table that stores information about students. We must be able to identify a student and store data associated with that individual student. Each student must have a row and only one row in the STUDENTS table. Also, each row in the STUDENTS table should identify one, and only one, student. The primary key mechanism enables an application, for example, to properly process student tuition bills. Every student will get one, and only one, tuition ...
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