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.

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SQL DEFAULT Constraint on CREATE TABLE

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

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT 'Mumbai'
)

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()
)

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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:

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Mumbai'

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To DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT