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
( Id int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City varchar(255) DEFAULT 'TVL' ) |
The DEFAULT constraint can also be used to insert system values,
by using functions like GETDATE():
CREATE TABLE Orders
( Id int NOT NULL, OrderNo int NOT NULL, 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 'TVL' |
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'TVL' |
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 |
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