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By default, a column can hold NULL values
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 when the "Persons" table is created:
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255)
NOT NULL,
Age int
);
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;