NULL and NOT NULL columns

If the column value is mandatory for a row, you define the column as being NOT NULL. Otherwise, the column is allowed to contain the NULL value, which represents no value. The default is to allow NULL values, but you should explicitly declare columns NOT NULL unless there is a good reason to allow NULL values.

The sample database has a table called Departments, which has columns named DepartmentID, DepartmentName, and DepartmentHeadID. Its definition is as follows:

Column	Data type	Size	NULL/NOT NULL	Constraint
DepartmentID	integer	—	NOT NULL	Primary key
DepartmentName	char	40	NOT NULL	None
DepartmentHeadID	integer	—	NULL	Foreign key

If you specify NOT NULL, a column value must be supplied for every row in the table.

NULL and NOT NULL columns

See also