Difference between Function and Procedure in SQL

When working with SQL, two common types of subprograms you’ll come across are Functions and Procedures. Both are used to perform specific tasks and encapsulate SQL code, but they serve different purposes and have important differences. Let's explore these in detail to help you understand when and why you would use each.

What is a Function in SQL?

A Function in SQL is a named subprogram designed to perform a single task, return a value, and can be used in SQL queries. Functions are often used when you need to calculate or return a value from given input parameters.

Key Features of SQL Functions:

Returns a Value: Functions always return a single value (scalar) or a table (table-valued function).

Called in SQL Queries: You can invoke a function in SQL statements like SELECT, INSERT, UPDATE, or DELETE.

Must Return a Value: A function must always return some value. You cannot write a function that doesn’t return anything.

Use in Expressions: Functions can be used as part of expressions, and their return values are processed within SQL queries.

Parameters: Functions typically take input parameters and always return a result based on those inputs.

Syntax Example of a Function:

CREATE FUNCTION GetEmployeeFullName (@EmployeeID INT)
RETURNS VARCHAR(100)
AS
BEGIN
    DECLARE @FullName VARCHAR(100)
    SELECT @FullName = FirstName + ' ' + LastName
    FROM Employees
    WHERE EmployeeID = @EmployeeID
    RETURN @FullName
END

What is a Procedure in SQL?

A Procedure in SQL (also known as Stored Procedure) is a named subprogram that performs one or more tasks but may or may not return a value. Procedures are used when you want to execute a series of SQL statements that might not necessarily need to return a result.

Key Features of SQL Procedures:

May or May Not Return a Value: Unlike functions, procedures can return zero, one, or more values using output parameters.

Cannot be Called in Queries: Procedures cannot be directly called in SQL queries like functions. They are executed using the EXEC or CALL command.

Perform Multiple Tasks: Procedures can execute multiple SQL statements, perform updates, or even manage transactions.

Parameters: Procedures can have input, output, or both types of parameters.

Complex Logic: Procedures can handle more complex operations compared to functions, such as working with transactions or executing different SQL commands.

Syntax Example of a Procedure:

CREATE PROCEDURE GetEmployeeDetails (@EmployeeID INT)
AS
BEGIN
    SELECT FirstName, LastName, Department, Salary
    FROM Employees      
    WHERE EmployeeID = @EmployeeID 
END

Key Differences Between Functions and Procedures in SQL?

Feature	Function	Procedure
Returns a Value	Must return a single value or table	May or may not return values
Usage in Queries	Can be called in SQL queries	Cannot be used in SQL queries
Return Type	Must return a value (scalar/table)	Can return multiple values using output parameters
Execution	Invoked by SQL statement (e.g., SELECT)	Executed using EXEC or CALL
Parameters	Input parameters only	Input, Output, or Both
Transaction Control	Functions do not manage transactions	Procedures can start, commit, or rollback transactions
Purpose	Used to compute and return a value	Used to perform tasks like CRUD operations, transaction management
Side Effects	Should not have side effects (like updating a table)	Can have side effects such as modifying data in tables

When to Use a Function vs a Procedure?

• Use a Function When: 
  

• You need to return a single value (e.g., calculation or formatting).
• You want to use the result in a SQL statement like SELECT or WHERE clauses.
• The operation is simple and doesn't involve changing the database state (no updates or deletes).
  
  
• Use a Procedure When: 
  
• You need to perform multiple SQL operations (like INSERT, UPDATE, DELETE).
• You want to manage transactions.
• You need to execute more complex business logic.
• You need to return multiple values using output parameters.

Conclusion

In SQL, Functions and Procedures play distinct roles. Functions are used for calculations and returning values within queries, while Procedures are more versatile and can perform a variety of operations, including modifying data. Understanding the differences between them allows you to pick the right tool for the job based on your specific use case.

Make sure to choose Functions when you need a result that can be embedded within a SQL statement and Procedures when your task involves multiple operations or complex logic.