Differences Between Stored Procedures and Functions

Stored procedures and functions are both reusable blocks of SQL logic, but they serve different purposes and behave differently in terms of execution, integration, and capabilities.

1. How They Are Called

• Stored Procedure: Called using the CALL statement, e.g., CALL update_inventory();. It runs as a standalone operation and is typically used to perform a series of actions like data manipulation, logging, or batch processing.
• Function: Invoked directly within SQL expressions, such as SELECT calculate_tax(price) FROM orders;. Functions are designed to return a value and integrate seamlessly into queries.

2. Return Values

• Stored Procedure: Can return multiple values using OUT or INOUT parameters. It doesn’t require a return value and is often used for operations that affect the database state or produce multiple outputs.
• Function: Must return exactly one value using the RETURN statement. This value is used within SQL expressions and must be deterministic and side-effect-free.

3. Use in SQL Queries

• Stored Procedure: Cannot be embedded inside SQL expressions like SELECT or WHERE. It must be executed independently and is often used for tasks like batch updates or report generation.
• Function: Can be used inline within SQL statements, making it ideal for calculations, formatting, or conditional logic directly within queries.

4. Data Modification Capabilities

• Stored Procedure: Can execute DML operations such as INSERT, UPDATE, DELETE, and MERGE. This makes procedures suitable for workflows that change the database state.
• Function: Typically read-only. Most SQL dialects restrict functions from modifying data to ensure they remain pure and side-effect-free.

5. Parameter Support

• Stored Procedure: Supports IN, OUT, and INOUT parameters.
• Function: Only supports IN parameters. It receives input, processes it, and returns a result.

6. Transaction Control

• Stored Procedure: Can include transaction control statements like START TRANSACTION, COMMIT, and ROLLBACK. This allows procedures to manage complex transactional logic.
• Function: Cannot manage transactions. They are expected to be atomic and side-effect-free, especially when used inside queries.

7. Exception Handling

• Stored Procedure: Supports robust error handling using DECLARE CONTINUE HANDLER or DECLARE EXIT HANDLER. This allows graceful recovery from runtime errors.
• Function: Has limited or no support for exception handling. Errors typically propagate to the calling query.

8. Compilation Behavior

• Stored Procedure: Compiled once and stored in the database. This improves performance for repeated calls and reduces overhead.
• Function: Often compiled at runtime each time it is invoked, especially when used inside queries.

9. Result Sets

• Stored Procedure: Can return multiple result sets, making it suitable for complex reporting or multi-step queries.

• Function: Returns a single scalar value. Some SQL dialects (e.g., PostgreSQL) support table-returning functions, but this is not universal.

10. Use of Temporary Tables

• Stored Procedure: Can create and manipulate temporary tables, which are useful for intermediate data processing or staging.
• Function: Typically restricted from using temporary tables. They rely on expressions and internal logic only.