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:
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() )

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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:
MySQL:

ALTER TABLE Persons ALTER City SET DEFAULT 'SANDNES'

SQL Server / Oracle / MS Access:

ALTER TABLE Persons ALTER COLUMN City SET DEFAULT 'SANDNES'

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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