SQL DEFAULT Constraint

SQL DEFAULT Constraint : SQL DEFAULT Constraint is used to provide a default value for a column.

• The default value will be added to all new records IF no other value is specified.

SQL DEFAULT on CREATE TABLE

The following SQL sets a DEFAULT value for the “City” column when the “Persons” table is created:

 

My SQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons (
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 GETDATE():

CREATE TABLE Orders (
ID int NOT NULL,
OrderNumber int NOT NULL,
OrderDate date DEFAULT GETDATE()
);

SQL DEFAULT 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’;

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;