NOT NULL constraints

NOT NULL constraints prevent null values from being entered into a column.

The null value is used in databases to represent an unknown state. By default, all of the built-in data types provided with the database manager support the presence of null values. However, some business rules might dictate that a value must always be provided (for example, every employee is required to provide emergency contact information). The NOT NULL constraint is used to ensure that a given column of a table is never assigned the null value. Once a NOT NULL constraint has been defined for a particular column, any insert or update operation that attempts to place a null value in that column will fail.

Because constraints only apply to a particular table, they are usually defined along with a table's attributes, during the table creation process. The following CREATE TABLE statement shows how the NOT NULL constraint would be defined for a particular column:

    CREATE TABLE EMPLOYEES    (. . . 
                               EMERGENCY_PHONE    CHAR(14)  NOT NULL,
                               . . .
                               );