MySQL DEFAULT Constraint
MySQL DEFAULT Constraint
The DEFAULT constraint is used to set a default value for a column.
The default value will be added to all new records, if no other value is specified.
DEFAULT on CREATE TABLE
The following SQL sets a DEFAULT value for the "City" column when the "Persons" table is created:
CREATE TABLE Persons
(
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
City varchar(255) DEFAULT 'Sandnes'
);
ID int NOT NULL,
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() :
CREATE TABLE Orders
(
ID int NOT NULL,
OrderNumber int NOT NULL,
OrderDate date DEFAULT CURRENT_DATE()
);
ID int NOT NULL,
OrderNumber int NOT NULL,
OrderDate date DEFAULT CURRENT_DATE()
);
DEFAULT on ALTER TABLE
To create 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';
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;
ALTER City DROP DEFAULT;