SQL DEFAULT Constraint

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# 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 / Oracle / MS Access:

    ALTER TABLE Persons
    ALTER COLUMN City SET 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

From http://www.w3schools.com (Copyright Refsnes Data)