SQL NOT NULL constraints
By default, the table's columns accept NULL values.
The NOT NULL constraint forces columns not to accept NULL values.
The NOT NULL constraint forces the field to always contain values. This means that if you do not add a value to a field, you cannot insert a new record or update a record.
The following SQL forces the P_Id" and "LastName" columns not to accept NULL values:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)