SQL : Sql Query Execution Order

Most programming languages executes set of statements from Top to Bottom but SQL Server executes set of statements in a logical order which is uniquely decides by the SQL Server in a predefined order known as Logical Query processing phase.

Introduction

When we are developing an application our concern mostly related to the code optimization which enhances the performance of application. When we are talking about an application it has broadly divided into two part one is the our code written in any programming language and other is the code written in database which executes at database level and return the results to our application.
In real world most application facing the performance issue which can be improved by code optimization. Here we will talk about the SQL query optimization.Most programming languages executes set of statements from Top to Bottom but SQL Server executes set of statements in a logical order which is uniquely decides by the SQL Server in a predefined order known as Logical Query processing phase.

Difference between Stored Procedures and User Defined Functions in SQL Server

Stored Procedure
Stored Procedures are pre-compile objects which are compiled for first time and saved, which executes (compiled code) whenever it is called. Stored procedure does not necessarily return data, but it can return more than one result set. It can be used to affect data (update/insert/delete), can have transactions, can have error handling, do not have restrictions about non-deterministic functions, and can call other stored procedures.

User Defined Function
User defined function is compiled and executed every time when it is called. Function attempts to return something, always, and has several restrictions - for example, you can't use DML statements, call stored procedures, call NEWID(), etc. You also cannot have error handling, transactions, or non-deterministic functions (e.g. GETDATE() in SQL Server 2000).

Differences :

• Stored procedures compiled first time and reuse the execution plan when next time called while function compiled every time when called.
• Stored procedure can have input parameter as well as output parameter while user defined function can have only input parameters.