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SQL DEFAULT Constraint

SQL DEFAULT Constraint

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

If no other value is specified, the default value will be added to all new records.

SQL DEFAULT Constraint When Creating a 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 When Altering a Table

When a table has already been created, to create a DEFAULT constraint on the "City" column, use the following SQL:

MySQL:

ALTER TABLE Persons
ALTER City SET DEFAULT 'SANDNES'

SQL Server / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT ab_c DEFAULT 'SANDNES' for City

Oracle:

ALTER TABLE Persons
MODIFY City DEFAULT 'SANDNES'

Dropping 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
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