What is NOT NULL Constraint?
The NOT NULL constraint enforces a column to NOT accept NULL values.The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field.
CREATE TABLE items ( item_num INTEGER, manufcode CHAR(3) NOT NULL, promotype INTEGER, descrip CHAR(20))
the Not Null constraint can be declared to a column in which you are not supposed to enter a null vale i.e you can't leave it as blank. If you had declare an column with not null then you must enter a value.
Create table stud( sid number not null, sname varchar2(20) not null, pno number);
While inserting values
101 praveen ;//accepted rajesh 1234568;//not accepted throws an error 101 123456;//not accepted throws an error