By default, a column can hold NULL value. The SQL NOT NULL Constraint is used to force a column cannot accept a NULL value, and must hold a value.
CREATE TABLE Users ( UserID int NOT NULL, FirstName varchar(100) NOT NULL, LastName varchar(100) NOT NULL, Email varchar(200), Phone varchar(50), Birthday Date )
The above script will create a table named “Users”, the column UserID, FirstName and LastName cannot hold NULL value. If you try to insert a row without specify the UserID, FirstName or LastName:
INSERT INTO Users (UserID, FirstName, Email, Phone, Birthday) VALUES (1, 'Tom', 'firstname.lastname@example.org', '999-9999', '1992-12-12')
You would get an error, because the LastName is not specified and would be set a NULL value.