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.
The following SQL ensures that the “ID”, “LastName”, and “FirstName” columns will NOT accept NULL values:
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255) NOT NULL,
Tip: If the table has already been created, you can add a NOT NULL constraint to a column with the ALTER TABLE statement.