SQL DEFAULT Constraint

SQL DEFAULT Constraint

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.

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()
); 

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:

ALTER TABLE Persons
ADD CONSTRAINT df_City
DEFAULT 'Sandnes' FOR City; 

MS Access:

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Sandnes'; 

Oracle:

ALTER TABLE Persons
MODIFY City DEFAULT 'Sandnes'; 

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;