SQL DEFAULT Constraint

SQL DEFAULT Constraint

The DEFAULT constraint is used to insert a default value into a column.

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

________________________________________

SQL DEFAULT Constraint on CREATE TABLE

The following SQL creates a DEFAULT constraint on the "City" column when the "Persons" table is created:

My SQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(

P_Id int NOT NULL,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255) DEFAULT 'Sandnes'

)

The DEFAULT constraint can also be used to insert system values, by using functions like GETDATE():

CREATE TABLE Orders

(

O_Id int NOT NULL,

OrderNo int NOT NULL,

P_Id int,

OrderDate date DEFAULT GETDATE()

)

________________________________________

SQL DEFAULT Constraint on ALTER TABLE

To create 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 / MS Access:

ALTER TABLE Persons

ALTER COLUMN City SET DEFAULT 'SANDNES'

Oracle:

ALTER TABLE Persons

MODIFY City DEFAULT 'SANDNES'

________________________________________

To DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:

MySQL:

ALTER TABLE Persons

ALTER City DROP DEFAULT

SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ALTER COLUMN City DROP DEFAULT