Friday, 15 February 2013

What is the SQL DEFAULT Constraint Syntax and Example

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 "Customers" table is created:

My SQL / SQL Server / Oracle / MS Access:
CREATE TABLE Customers
(
C_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT 'Hyderabad'
)

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,
C_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 Customers
ALTER City SET DEFAULT 'Hyderabad'

SQL Server / MS Access:
ALTER TABLE Customers
ALTER COLUMN City SET DEFAULT 'Hyderabad'

Oracle:
ALTER TABLE Customers
MODIFY City DEFAULT 'Hyderabad'
To DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:
MySQL:
ALTER TABLE Customers
ALTER City DROP DEFAULT

SQL Server / Oracle / MS Access:
ALTER TABLE Customers
ALTER COLUMN City DROP DEFAULT

Thanks,
TAMATAM

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