NOT NULL Constraint in SQL

NOT NULL constraint makes sure that a column does not hold NULL value. When we don’t provide value for a particular column while inserting a record into a table, by default it takes NULL value. By specifying NULL constraint, we can be sure that a particular column(s) cannot have NULL values.

How to specify the NULL constraint while creating table

Here I am creating a table “STUDENTS”. I have specified NOT NULL constraint for columns ROLL_NO, STU_NAME and STU_AGE which means you must provide the value for these three fields while inserting/updating records in this table. It enforces these column(s) not to accept null values.

CREATE TABLE STUDENTS(
       ROLL_NO   INT           NOT NULL,
       STU_NAME VARCHAR (35)   NOT NULL,
       STU_AGE  INT            NOT NULL,
       STU_ADDRESS  VARCHAR (235) ,     
       PRIMARY KEY (ROLL_NO)
);

Specify the NULL constraint for already existing table

In the above section we learnt how to specify the NULL constraint while creating a table. However we can specify this constraint on a already present table also. For this we need to use ALTER TABLE statement.

ALTER TABLE STUDENTS
   MODIFY STU_ADDRESS  VARCHAR (235) NOT NULL;

After this STU_ADDRESS column will not accept any null values.