Using the NOT NULL Constraint

Use the NOT NULL keywords to require that a column receive a value during insert or update operations. If you place a NOT NULL constraint on a column (and no default value is specified), you must enter a value into this column when you insert a row or update that column in a row. If you do not enter a value, the database server returns an error, because no default value exists.

The following example creates the newitems table. In newitems, the column manucode does not have a default value nor does it allow NULL values.

CREATE TABLE newitems (
   newitem_num INTEGER,
   manucode CHAR(3) NOT NULL,
   promotype INTEGER,
   descrip CHAR(20)) 

You cannot specify NULL as the explicit default value for a column if you also specify the NOT NULL constraint.