SQL NOT NULL Constraint

SQL NOT NULL Constraint:

By default, a column can hold NULL values.

The NOT NULL constraint enforces a column to NOT accept NULL values.

This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.

SQL NOT NULL on CREATE TABLE

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

SQL NOT NULL on ALTER TABLE

ALTER TABLE Persons MODIFY Age int NOT NULL;