The usage and examples of SQL NOT NULL keyword

SQL Keyword Reference

NOT NULL

NOT NULL constraint enforces that the column does not accept NULL values, which means you must insert or update a record without adding a value to the field.

The following SQL ensures that the "ID", "LastName", and "FirstName" column will not accept NULL values:

CREATE TABLE Persons (
　ID int NOT NULL,　
　　LastName varchar(255) NOT NULL,
　FirstName varchar(255) NOT NULL,
　Age int
);

When the 'Persons' table has been created, the following SQL can create a NOT NULL constraint on the 'age' column:

ALTER TABLE Persons　
MODIFY Age int NOT NULL;

SQL Keyword Reference