MySQL Default

MySQL DEFAULT Constraint

The DEFAULT constraint is used to automatically insert a default value for a column, if no value is specified.

The default value will be added to all new records (if no other value is specified).

DEFAULT Constraint on CREATE TABLE

The following SQL sets a DEFAULT value for the "City" column upon creation of the "Persons" table:

CREATE TABLE Persons
(
    ID int PRIMARY KEY,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Age int,

City varchar(255) DEFAULT 'Sandnes'
);

The DEFAULT constraint can also be used to insert system values, by using functions like CURRENT_DATE() to insert the current date:

CREATE TABLE Orders
(
    ID int PRIMARY KEY,

OrderNumber int NOT NULL,

OrderDate date DEFAULT CURRENT_DATE()
);

DEFAULT Constraint on ALTER TABLE

To define a DEFAULT constraint on the "City" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ALTER City SET DEFAULT 'Sandnes';

DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:

ALTER TABLE Persons
ALTER City DROP DEFAULT;