MySQL NOT NULL Constraint
MySQL 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.
NOT NULL on CREATE TABLE
The following SQL ensures that the "ID", "LastName", and "FirstName" columns will NOT accept NULL values when the "Persons" table is created:
ExampleGet your own SQL Server
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255)
NOT NULL,
Age int
);
NOT NULL on ALTER TABLE
To create a NOT NULL
constraint on the "Age" column when the "Persons" table is already created, use the following SQL:
Example
ALTER TABLE Persons
MODIFY Age int NOT NULL;
Exercise?What is this?
Test your skills by answering a few questions about the topics of this page
What is the purpose of the SQL NOT NULL
constraint?