This article describes the differences between Stored Procedures and User Defined Functions in SQL Server.
A Stored Procedure is nothing more than prepared SQL code that you save so you can reuse the code over and over again. So if you think about a query that you write over and over again, instead of having to write that query each time you would save it as a Stored Procedure and then just call the Stored Procedure to execute the SQL code that you saved as part of the Stored Procedure.
In addition to running the same SQL code over and over again you also have the ability to pass parameters to the Stored Procedure, so depending on what the need is, the Stored Procedure can act accordingly based on the parameter values that were passed.
Stored Procedures can also improve performance. Many tasks are implemented as a series of SQL statements. Conditional logic applied to the results of the first SQL statements determine which subsequent SQL statements are executed. If these SQL statements and conditional logic are written into a Stored Procedure, they become part of a single execution plan on the server. The results do not need to be returned to the client to have the conditional logic applied; all of the work is done on the server.
Benefits of Stored Procedures
• Pre-compiled execution: SQL Server compiles each Stored Procedure once and then re-utilizes the execution plan. This results in tremendous performance boosts when Stored Procedures are called repeatedly.
• Reduced client/server traffic: If network bandwidth is a concern in your environment then you’ll be happy to learn that Stored Procedures can reduce long SQL queries to a single line that is transmitted over the wire.
• Efficient reuse of code and programming abstraction: Stored Procedures can be used by multiple users and client programs. If you utilize them in a planned manner then you’ll find the development cycle requires less time.
• Enhanced security controls: You can grant users permission to execute a Stored Procedure independently of underlying table permissions.
User Defined Functions
Like functions in programming languages, SQL Server User Defined Functions are routines that accept parameters, perform an action such as a complex calculation, and returns the result of that action as a value. The return value can either be a single scalar value or a result set.
Functions in programming languages are subroutines used to encapsulate frequently performed logic. Any code that must perform the logic incorporated in a function can call the function rather than having to repeat all of the function logic.
SQL Server supports two types of functions
• Built-in functions: Operate as defined in the Transact-SQL Reference and cannot be modified. The functions can be referenced only in Transact-SQL statements using the syntax defined in the Transact-SQL Reference.
• User Defined Functions: Allow you to define your own Transact-SQL functions using the CREATE FUNCTION statement. User Defined Functions use zero or more input parameters, and return a single value. Some User Defined Functions return a single, scalar data value, such as an int, char, or decimal value.
Benefits of User Defined Functions
• They allow modular programming: You can create the function once, store it in the database, and call it any number of times in your program. User Defined Functions can be modified independently of the program source code.
• They allow faster execution: Similar to Stored Procedures, Transact-SQL User Defined Functions reduce the compilation cost of Transact-SQL code by caching the plans and reusing them for repeated executions. This means the user-defined function does not need to be reparsed and reoptimized with each use resulting in much faster execution times. CLR functions offer significant performance advantage over Transact-SQL functions for computational tasks, string manipulation, and business logic. Transact-SQL functions are better suited for data-access intensive logic.
• They can reduce network traffic: An operation that filters data based on some complex constraint that cannot be expressed in a single scalar expression can be expressed as a function. The function can then invoked in the WHERE clause to reduce the number or rows sent to the client.
Difference between a User Defined Function and a Stored Procedure
1. Function must return a value. Stored Procedure may or not return values.
2. Function allows only Select statements, it will not allow us to use DML statements. Stored Procedure can have select statements as well as DML statements such as insert, update, delete and so on.
3. Function allows only input parameters, doesn’t support output parameters. Stored Procedure can have both input and output parameters.
4. Function does not allow us to use try-catch blocks. In the Stored Procedure we can use try catch blocks for exception handling.
5. Transactions are not allowed within functions. We can use transactions within Stored Procedures.
6. In the function we can use only table variables, it will not allow using temporary tables. In Stored Procedure we can use both table variables as well as temporary table in it.
7. Stored Procedures can’t be called from a function. Stored Procedures can call functions.
8. Functions can be called from a select statement. Procedures can’t be called from Select/Where/Having and so on statements. Execute/Exec statement can be used to call/execute Stored Procedure.
9. A UDF can be used in join clause as a result set. Procedures can’t be used in Join clause.
10. Stored Procedures are pre-compile objects which are compiled for first time and its compiled format is saved which executes (compiled code) whenever it is called. But Function is compiled and executed every time when it is called.
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