The CREATE DEFAULT statement in SQL is used to define a default value for a column within a table or a user-defined data type. However, it is important to note that the CREATE DEFAULT syntax has been deprecated in some database systems (like Microsoft SQL Server) in favor of using constraints, specifically the DEFAULT constraint.
That said, if you're aiming to define a default value for an existing column in a table, the most common approach is to use the ALTER TABLE statement to add a default constraint. Here’s how both methods can work depending on the database system you are using:
Using CREATE DEFAULT (legacy method):
CREATE DEFAULT MyDefaultValue AS 'DefaultStringValue';
Once you've created a default, you would typically later associate it with a column using an ALTER TABLE statement. For example:
ALTER TABLE MyTable
ADD CONSTRAINT MyDefaultConstraint
DEFAULT MyDefaultValue FOR MyColumn;
Using ALTER TABLE with DEFAULT (recommended method):
The modern approach (and recommended) is to use the ALTER TABLE statement directly to set a default value. Here’s how you can do that:
ALTER TABLE MyTable
ADD CONSTRAINT MyDefaultConstraint
DEFAULT 'DefaultStringValue' FOR MyColumn;
Example: Adding a default value
Suppose you have a table named Employees with a column Status. You want to set a default value of 'Active' to the Status column. You can do it like this:
ALTER TABLE Employees
ADD CONSTRAINT DF_Employees_Status
DEFAULT 'Active' FOR Status;
Removing a Default Value
If at any point you need to remove the default value, you can drop the constraint:
ALTER TABLE Employees
DROP CONSTRAINT DF_Employees_Status;
Note:
- The exact syntax and availability of default constraints may vary slightly depending on the specific SQL database platform being used (for example, SQL Server, PostgreSQL, MySQL, etc.).
- The CREATE DEFAULT statement is generally not used in modern SQL practices and should usually be avoided in favor of using ALTER TABLE with a DEFAULT constraint.
Always refer to your specific SQL database documentation for the most accurate and detailed information on syntax and features.