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The DEFAULT constraint is used to set a default value for a column
The DEFAULT constraint is used to set a default value for a column.
The default value will be added to all new records, if no other value is specified.
The following SQL sets a DEFAULT value for the "City" column when the "Persons" table is created:
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()
);
To create a DEFAULT constraint on the "City" column when the table is already created, use the following SQL:
ALTER TABLE Persons
ALTER City SET DEFAULT 'Sandnes';
ALTER TABLE Persons
ADD CONSTRAINT df_City
DEFAULT 'Sandnes' FOR City;
ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Sandnes';
ALTER TABLE Persons
MODIFY City DEFAULT 'Sandnes';
To drop a DEFAULT constraint, use the following SQL:
ALTER TABLE Persons
ALTER City DROP DEFAULT;
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;