Not Null
The NOT NULL constraint enforces a column to not accept NULL values, which means that you cannot insert or update a record without adding a value to this field.
The following SQL ensures that the "ID", "LastName", and "FirstName" columns will NOT accept NULL values:
Example
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255)
NOT NULL,
Age int
);The following SQL creates a NOT NULL constraint on the "Age" column when the "Persons" table is already created:
ALTER TABLE Persons
MODIFY Age int NOT NULL;