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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.

Example
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
To create a NOT NULL constraint on the "Age" column when the "Persons" table is already created, use the following SQL:

ALTER TABLE Persons
MODIFY Age int NOT NULL;