By default all columns in a table can contain null values. If you want to ensure that a column must always have a value, i.e. it should not be left blank, then define a NOT NULL constraint on it.
The SQL 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.
Syntax: (Column_name [data type] [NOT NULL],
(Column_name data type [Constraint <name>] NOT NULL,
Query: The following SQL enforces the "P_Id" column and the "LastName" column to not accept NULL values:
SQL> CREATE TABLE Persons
P_Id number (6) NOT NULL,
LastName varchar2 (15) NOT NULL,
FirstName varchar2 (15),
Address varchar2 (25),
City varchar2 (15)
Note : In Persons table some persons might have P_Id and some employees might not have any person Id. If you put NOT NULL constraint on P_Id column then you will not be able insert rows for those persons whose P_Id is null.
Query: In the following table enforce the Name column not to accept null values.
SQL) CREATE TABLE AA
(NAME VARCHAR2(20) CONSTRAINT NN_AA NOT NULL);
SQL) DESC AA;