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:
MySQL
CREATE TABLE Orders
(
ID int PRIMARY KEY,
OrderNumber int NOT NULL,
OrderDate date DEFAULT CURRENT_DATE()
);SQL Server
To achieve the same result in SQL Server use the following SQL (to insert the current date):
CREATE TABLE Orders
(
ID int PRIMARY KEY,
OrderNumber int NOT NULL,
OrderDate date DEFAULT CAST(GETDATE() AS 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:
MySQL
ALTER TABLE Persons
ALTER City SET DEFAULT 'Sandnes';SQL Server
ALTER TABLE Persons
ADD CONSTRAINT df_City DEFAULT 'Sandnes' FOR City;MS Access
ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Sandnes';Oracle
ALTER TABLE Persons
MODIFY City DEFAULT 'Sandnes';Drop a DEFAULT Constraint
To drop a DEFAULT constraint, use the following SQL:
MySQL
ALTER TABLE Persons
ALTER City DROP DEFAULT;SQL Server
ALTER TABLE Persons
DROP CONSTRAINT df_City;MS Access
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;Oracle
ALTER TABLE Persons
MODIFY (City DEFAULT NULL);