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Difference between CTE and Temp Table and Table Variable

Posted By : Shailendra Chauhan, 15 Sep 2013
Updated On : 04 Oct 2013
Total Views : 121,439   
 
Keywords : cte vs temp table vs table variable, local temp table vs global temp table, when to use cte and temp table and table variable

Temp Table or Table variable or CTE are commonly used for storing data temporarily in SQL Server. In this article, you will learn the differences among these three.

CTE

CTE stands for Common Table expressions. It was introduced with SQL Server 2005. It is a temporary result set and typically it may be a result of complex sub-query. Unlike temporary table its life is limited to the current query. It is defined by using WITH statement. CTE improves readability and ease in maintenance of complex queries and sub-queries. Always begin CTE with semicolon.

A sub query without CTE is given below :

SELECT * FROM (
 SELECT Addr.Address, Emp.Name, Emp.Age From Address Addr
 Inner join Employee Emp on Emp.EID = Addr.EID) Temp
WHERE Temp.Age > 50
ORDER BY Temp.NAME

By using CTE above query can be re-written as follows :

;With CTE1(Address, Name, Age)--Column names for CTE, which are optional
AS
(
SELECT Addr.Address, Emp.Name, Emp.Age from Address Addr
INNER JOIN EMP Emp ON Emp.EID = Addr.EID
)
SELECT * FROM CTE1 --Using CTE 
WHERE CTE1.Age > 50
ORDER BY CTE1.NAME

When to use CTE

  1. This is used to store result of a complex sub query for further use.

  2. This is also used to create a recursive query.

Temporary Tables

In SQL Server, temporary tables are created at run-time and you can do all the operations which you can do on a normal table. These tables are created inside Tempdb database. Based on the scope and behavior temporary tables are of two types as given below-

  1. Local Temp Table

    Local temp tables are only available to the SQL Server session or connection (means single user) that created the tables. These are automatically deleted when the session that created the tables has been closed. Local temporary table name is stared with single hash ("#") sign.

    CREATE TABLE #LocalTemp
    (
     UserID int,
     Name varchar(50), 
     Address varchar(150)
    )
    GO
    insert into #LocalTemp values ( 1, 'Shailendra','Noida');
    GO
    Select * from #LocalTemp
    

    The scope of Local temp table exist to the current session of current user means to the current query window. If you will close the current query window or open a new query window and will try to find above created temp table, it will give you the error.

  2. Global Temp Table

    Global temp tables are available to all SQL Server sessions or connections (means all the user). These can be created by any SQL Server connection user and these are automatically deleted when all the SQL Server connections have been closed. Global temporary table name is stared with double hash ("##") sign.

    CREATE TABLE ##GlobalTemp
    (
     UserID int,
     Name varchar(50), 
     Address varchar(150)
    )
    GO
    insert into ##GlobalTemp values ( 1, 'Shailendra','Noida');
    GO
    Select * from ##GlobalTemp
     

    Global temporary tables are visible to all SQL Server connections while Local temporary tables are visible to only current SQL Server connection.

Table Variable

This acts like a variable and exists for a particular batch of query execution. It gets dropped once it comes out of batch. This is also created in the Tempdb database but not the memory. This also allows you to create primary key, identity at the time of Table variable declaration but not non-clustered index.

 GO
 DECLARE @TProduct TABLE
 (
 SNo INT IDENTITY(1,1),
 ProductID INT,
 Qty INT
 ) 
 --Insert data to Table variable @Product 
 INSERT INTO @TProduct(ProductID,Qty)
 SELECT DISTINCT ProductID, Qty FROM ProductsSales ORDER BY ProductID ASC 
 --Select data
 Select * from @TProduct
 
 --Next batch
 GO
 Select * from @TProduct --gives error in next batch
 

Note

  1. Temp Tables are physically created in the Tempdb database. These tables act as the normal table and also can have constraints, index like normal tables.

  2. CTE is a named temporary result set which is used to manipulate the complex sub-queries data. This exists for the scope of statement. This is created in memory rather than Tempdb database. You cannot create any index on CTE.

  3. Table Variable acts like a variable and exists for a particular batch of query execution. It gets dropped once it comes out of batch. This is also created in the Tempdb database but not the memory.

What do you think?

I hope you will enjoy the tips while programming with SQL Server. I would like to have feedback from my blog readers. Your valuable feedback, question, or comments about this article are always welcome.

 
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About the Author
Hey! I'm Shailendra Chauhan author, developer with more than 5 years of hand over Microsoft .NET technologies. I am a .NET Consultant, founder & chief editor of www.dotnet-tricks.com and www.webgeekschool.com. I am author of books ASP.NET MVC Interview Questions and Answers & LINQ Interview Questions and Answers.
I love to work with web applications and mobile apps using Microsoft technology including ASP.NET, MVC, C#, SQL Server, WCF, Web API, Entity Framework,Cloud Computing, Windows Azure, jQuery, jQuery Mobile, Knockout.js, Angular.js and many more web technologies. More...
 
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