SQL Default Constraint
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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.
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() ) |
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’ |
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 |