The NOT NULL constraint enforces a field or column to not contain a null value.

Example of NOT NULL constraint with CREATE TABLE statement:

CREATE TABLE PERSONS (
 
P_ ID INT NOT NULL,
 
NAME VARCHAR (20) NOT NULL,
 
AGE INT,
 
PRIMARY KEY (P_ID)
 
);

Example of NOT NULL constraint with ALTER TABLE statement:

ALTER TABLE PERSONS MODIFY AGE INT NOT NULL;