SQL DEFAULT Constraint

SQL DEFAULT Constraint

The 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.

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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()
);

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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';

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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;