Difference Between Stored Procedure and User Defined Function in SQL Server

Stored Procedure

• Purpose:

  • Stored procedures are used to perform a series of actions such as inserting, updating, deleting records, and complex calculations.

• Syntax:

  CREATE PROCEDURE procedure_name AS BEGIN -- SQL statements END

• Return Type:

  • Stored procedures do not have to return a value, but they can use OUTPUT parameters or return a status code using the RETURN keyword.

• Execution:

  • Executed using the EXEC or EXECUTE statement.

  EXEC procedure_name;

• Side Effects:

  • Stored procedures can have side effects like modifying data in tables.

• Transaction Control:

  • Stored procedures can include transaction control statements like BEGIN TRANSACTION, COMMIT, and ROLLBACK.

• Performance:

  • Stored procedures are precompiled, meaning the execution plan is created and stored in the cache upon first execution, which can lead to performance improvements on subsequent executions.

• Usage:

  • Generally used for encapsulating business logic, data manipulation, and complex operations.

User Defined Function (UDF)

• Purpose:

  • Functions are used to compute and return a single value (scalar function) or a table (table-valued function) based on the input parameters.

• Syntax:

  CREATE FUNCTION function_name (@param datatype) RETURNS return_datatype AS BEGIN -- SQL statements RETURN return_value; END

• Return Type:

  • Functions must return a value. Scalar functions return a single value, while table-valued functions return a table.

• Execution:

  • Invoked as part of a SQL statement.

  SELECT function_name(param);

• Side Effects:

  • Functions cannot modify data in tables or have side effects.

• Transaction Control:

  • Functions cannot contain transaction control statements like BEGIN TRANSACTION, COMMIT, or ROLLBACK.

• Performance:

  • Functions are executed as part of the SQL statement and do not benefit from precompilation like stored procedures.

• Usage:

  • Generally used for encapsulating reusable calculations and returning values that can be used within SQL queries.

Key Differences

Examples

Stored Procedure Example:

CREATE PROCEDURE GetCustomerOrders
    @CustomerID INT
AS
BEGIN
    SELECT OrderID, OrderDate, TotalAmount
    FROM Orders
    WHERE CustomerID = @CustomerID;
END;

User Defined Function Example:

Scalar Function:

CREATE FUNCTION GetCustomerOrderCount (@CustomerID INT)
RETURNS INT
AS
BEGIN
    DECLARE @OrderCount INT;
    SELECT @OrderCount = COUNT(*)
    FROM Orders
    WHERE CustomerID = @CustomerID;
    RETURN @OrderCount;
END;

Table-Valued Function:

CREATE FUNCTION GetCustomerOrders (@CustomerID INT)
RETURNS TABLE
AS
RETURN
(
    SELECT OrderID, OrderDate, TotalAmount
    FROM Orders
    WHERE CustomerID = @CustomerID
);

Stored procedures and user-defined functions serve different purposes and have distinct capabilities in SQL Server, making them useful for different scenarios in database development and management.