The ALTER FUNCTION statement is used in SQL to modify an existing function in a database. The specific syntax and options available may vary depending on the database management system (DBMS) you are using (like PostgreSQL, SQL Server, etc.). Below is a general outline of how to use the ALTER FUNCTION statement in SQL:
General Syntax
In many SQL database systems, the ALTER FUNCTION statement is used for operations such as changing the function's properties (like its volatility), or to change the RETURN type. However, not all DBMSs allow extensive modifications through this statement. For major changes like modifying the function body, you often have to drop the function and recreate it.
For example:
ALTER FUNCTION function_name (parameters)
[SET SCHEMA new_schema]
[ALTER | REPLACE] FUNCTION_body;
PostgreSQL Example
In PostgreSQL, you can use the ALTER FUNCTION statement to change certain characteristics of a function.
1. Changing the schema of the function:
ALTER FUNCTION old_schema.function_name(parameter_type)
SET SCHEMA new_schema;
2. Changing the VOLATILITY of a function:
ALTER FUNCTION function_name(parameter_type)
VOLATILE | STABLE | IMMUTABLE;
In PostgreSQL, if you need to change the function body or signature (like parameters), you typically have to drop the function and recreate it.
SQL Server Example
In SQL Server, you cannot modify the function directly using ALTER FUNCTION. Instead, you must redefine the entire function:
CREATE OR ALTER FUNCTION dbo.function_name (@parameter datatype)
RETURNS return_datatype
AS
BEGIN
-- Function logic here
RETURN value;
END;
Oracle Example
In Oracle PL/SQL, the syntax for altering a function is similar in that you often redefine it:
CREATE OR REPLACE FUNCTION function_name(parameter_name parameter_type)
RETURN return_type
AS
BEGIN
-- function implementation
END;
Summary
- Modify attributes or schema: Use ALTER FUNCTION for minor modifications (e.g., changing the schema).
- Modify function definition: For changes to parameters or the function body, use CREATE OR REPLACE FUNCTION in systems like PostgreSQL or Oracle.
- Refer to specific documentation: Always refer to the documentation for your specific DBMS to understand the capabilities and limitations of the ALTER FUNCTION statement and how to properly use it.