SQL Server - Temporary Tables

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.NET – Basic Terms in .Net »

SQL – Temporary Tables
September 27, 2007 by Prashant Pandey

Introduction
Temporary Tables are a great T-SQL feature that lets you store and process intermediate results by using the same selection, update, and join capabilities that you can use with typical SQL Server tables.

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typical SQL Server tables. “In the early days when I used Access I used to create tables that I treated as temporary and then delete them whenever I finished my task ”. Using SQL Server this is much simpler.”

Types of Temporary Tables in SQL
You can create two types of temporary tables in SQL, local and global temporary tables. The two types of temporary tables, local and global, differ from each other in their names, their visibility, and their availability.
Local Tem porary Tables

Local temporary tables prefix with single number sign (#) as the first character of their names, like (#table_name). Local temporary tables are visible only in the current session OR you can say that they are visible only to the current connection for the user. They are deleted when the user disconnects from instances of Microsoft SQL Server.
Global Tem porary Tables

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Global temporary tables prefix with double number sign (##) as the first character of their names, like (##table_name). Global temporary tables are visible to all sessions OR you can say that they are visible to any user after they are created. They are deleted when all users referencing the table disconnect from Microsoft SQL Server. For example: If you create a table named employees, the table can be used by any person who has the security permissions in the database to use it, until the table is deleted. If you create a local temporary table named #employees, you are the only person who can work with the table, and it is deleted when you disconnect. If you create a global temporary table named
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table, and it is deleted when you disconnect. If you create a global temporary table named ##employees, any user in the database can work with this table. If no other user works with this table after you create it, the table is deleted when you disconnect. If another user works with the table after you create it, SQL Server deletes it when both of you disconnect. SQL statements for creating the temporary table using the CREATE TABLE statement:
CREATE TABLE #MyTempTable (cola INT PRIMARY KEY) INSERT INTO #MyTempTable VALUES (1) SELECT * FROM #MyTempTable

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When you create local or global temporary tables, the CREATE TABLE syntax supports constraint definitions with the exception of FOREIGN KEY constraints. If a FOREIGN KEY constraint is specified in a temporary table, the statement returns a warning message indicating that the constraint was sk ipped, and the table is still created without the FOREIGN KEY constraints. Temporary tables cannot be referenced in FOREIGN KEY constraints.

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Naming Of Temporary Tables
Temporary tables are always created in tempdb. No matters it is created from the stored procedure internally or from the SQL Query Analyzer window. If a local temporary table is created in a stored procedure or application that can be executed at the same time by several users, SQL Server has to be able to distinguish the tables created by the different users. SQL Server does this by internally appending a numeric suffix to each local temporary table name. The full name of a temporary table as stored in the sysobjects table in tempdb consists of table name specified in the CREATE TABLE statement and the system-generated numeric suffix. To allow for the suffix, table_name specified for a local temporary name cannot exceed 116 characters. Reason: “Table names in SQL are the combination of owner.table_name and it must be unique within the database. Table_name can contain a maximum of 128 characters, except for
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local temporary table names (names prefixed with a single number sign (#)) that cannot exceed 116 characters.” For example: CREATE TABLE #Yaks (YakID int,YakName char(30)) SELECT Name FROM tempdb.dbo.sysobjects WHERE Name LIKE ‘#yak%’ The above select query will return the result something like below displayed. name ——————————————————————#Yaks__________________________________________________00000000001D (1 row(s) affected) SQL Server stores temporary tables as a database object with a type of unique number appended on the end of the name. But in case of temporary tables it may be the possibility that two users can create the temporary table with the same name as we know that all temporary tables stored in the tempdb database, so if the name of both temporary tables will be same then SQL server will through an error to the user because objects name in the SQL server must be unique in the database. So, to avoid this problem whenever anyone creates any temporary table SQL server automatically appends a unique string with the table name that is given by user while creating the table, But don’t worry you need to do anything with this extra string, It does all this for you automatically. You just have to refer to your #table_name with which you have created the temporary table. You can refer this name anywhere in your code where you want.

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Deletion of Temporary Tables
A local temporary table created in a stored procedure is dropped automatically when the stored procedure completes. The table can be referenced by any nested stored procedures executed by the stored procedure that created the table. The table cannot be referenced by the process which called the stored procedure that created the table. All other local temporary tables are dropped automatically at the end of the current session.
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current session. Global temporary tables are automatically dropped when the session that created the table ends and all other tasks have stopped referencing them. The association between a task and a table is maintained only for the life of a single Transact-SQL statement. This means that a global temporary table is dropped at the completion of the last Transact-SQL statement that was actively referencing the table when the creating session ended. For example: SQL statements for deleting the temporary table using the DROP TABLE statement: DROP TABLE #Table_name A local temporary table created within a stored procedure or trigger is distinct from a temporary table with the same name created before the stored procedure or trigger is called.

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Use of Temporary Table
Temporary tables are used in several ways. Most commonly uses to keep the result of a called stored procedure, to reduce the number of rows for joins, to aggregate data from different sources, or to replaces cursors and for parameterized views. SQL Server cursors have huge overhead. Maintenance of code is much easier if you use temporary tables to the T-SQL. It will be much easier to debug your stored procedure when your using temporary tables as the data will be saved in temporary tables.

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Limitations of Temporary Tables
Temporary tables are created in the tempdb database and create additional overhead for
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SQL Server, reducing overall performances. This happens because all reads and writes to the temporary tables are done within the tempdb database.

Using Temporary Tables Effectively
If you do not have any option other than to use temporary tables, use them effectively. There are few steps to be taken. Only include the necessary columns and rows rather than using all the columns and all the data which will not make sense of using temporary tables. Always filter your data into the temporary tables. When creating temporary tables, do not use SELECT INTO statements, as it places locks on system objects. Instead of SELECT INTO statements, create the table using Transact-SQL DDL statements and use INSERT INTO to populate the temporary table. Use indexes on temporary tables. Earlier days, I always forget to use an index on temporary. Specially, for large temporary tables consider using clustered and nonclustered indexes on temporary tables. You will have to test to see if indexes help or hurt overall performance. After you finish the using your temporary table, delete them. This will free the tempdb resources. Yes, I agree that temporary tables are deleted when connection is ended. But do not wait until such time. When creating a temporary table do not create them with a transaction. If you create it with a transaction, it will lock some system tables (syscolumns, sysindexes, syscomments). This will prevent others from executing the same query. In general, temp tables should be avoided, if possible. Because they are created in the tempdb database, they will create additional overhead for SQL Server, slowing overall
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performance. As an alternative to temp tables, consider the following alternatives: Rewrite your code so that the action you need completed can be done using a standard query or stored procedure, without using a temp table. Use a derived table. Consider using a correlated sub-query. Use a permanent table instead. Use a UNION statement to mimic a temp table. Use a table variable. It all depends on your requirement at that time when you are creating your query, but as written in Microsoft SQL Server Book Online, “Consider using table variables instead of temporary tables. Temporary tables are useful in cases when indexes need to be created explicitly on them, or when the table values need to be visible across multiple stored procedures or functions. In general, table variables contribute to more efficient query processing.” If you are using SQL Server 2000 or higher, you can take advantage of the new TABLE variable type. These are similar to temporary tables except with more flexibility and they always stay in memory. The code using a table variable might look like below: —————————————————————– DECLARE @TibetanYaks TABLE ( YakID int, YakName char(30) ) INSERT INTO @TibetanYaks (YakID, YakName) SELECT YakID, YakName
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FROM dbo.Yaks WHERE YakType = ‘Tibetan’ —————————————————————– Table variables don’t need to be dropped when you are done with them.

Which to use
If you have less than 100 rows generally use a table variable. Otherwise use a temporary table. This is because SQL Server won’t create statistics on table variables. If you need to create indexes on it then you must use a temporary table. When using temporary tables always create them and create any indexes and then use them. This will help reduce recompilations. The impact of this is reduced starting in SQL Server 2005 but it’s still a good idea.

Conclusion
Generally, temporary tables should be avoided as much as possible. If you need to use them follow the steps above so that you have the minimum impact on server performance.
*******

Reference from and many thanks to following websites and articles: Microsoft’s Book Online For SQL Server 2000 Sql-Server-Performance Sql Server Central SqlTeam
Prashant Pandey (TechAhead)   2 Votes 
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Posted in SQL | 28 C omments Like Be the first to like this post.

28 Responses
Rohan Patil
on October 1, 2007 at 5:43 pm | Reply

This is very usefull information for all computer users n programmers.Prashant Rocks!!!

mannu4mit Hey!! u are doing a gr8 work.

on October 16, 2007 at 1:08 pm | Reply

this one is useful place for all developers searching for latest .net/sql interview questions. even for the beginners who want to grassp knowledge… Best of luck dude… continue with this good work. Amit ……………….. open in browser customize free license contest

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rahul thnx it helped me very much

on October 30, 2007 at 12:04 pm | Reply

prashantvictory

on October 30, 2007 at 5:42 pm | Reply

Hi rahul, thanks for your comment.. and tell you one thing .. good comments always boost my confidence.. but bad comments or suggestion are also invited caz they told me to make myself more perfect in that area.. I pleased that this article helps you, So keep visiting the blog for new and intresting topics.. 

Uday

on October 31, 2007 at 5:33 am | Reply

Thank u very much for giving this much information.But will u add a little bit more about this

prashantvictory Hi Uday,

on October 31, 2007 at 5:56 am | Reply

I think i covered all about temporary tables , wht i think… But as you know no one is perfect, so tell me wht else i need to add in this article. tell me i’ll add it here, Your suggestions are always welcome…

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Ravi Hi Prashant,

on November 26, 2007 at 7:23 pm | Reply

Great work dude.Do more and more better next time..my best wishes are always with u. Ravi Raghav http://www.realitservices.com

Ravi

on November 26, 2007 at 7:31 pm | Reply

Real IT services specialises in providing Web and Software Development solutions in terms of the requirements like IT Outsourcing,CRM,Business Process Outsourcing,Content Management System,Offshore Software Development,Website Promotion(SEO),Customer Software Development at an affordable cost with 100% customer satisfaction.To know more about our services Go here http://www.realitservices.com

bhanu

on February 27, 2008 at 12:16 pm | Reply

i need to have questionnumber as an identity (int) in which my sp contains one parameter like this CREATE PROCEDURE spGetAnswerDescription1(@TestId int) AS create table #temptable (QID int identity, QuestionID int) open in browser customize free license contest

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QuestionID int) SELECT tbAnswers.AnswerDescription,tbQuestionPaper.ImagePath from tbQuestionPaper ,tbAnswers where tbQuestionPaper.QuestionId=tbAnswers.QuestionId and tbQuestionPaper.TestID=@TestId select * from #temptable GO when dis compiled i m getting an error tht i cant insert when identity_insert is OFF when i create SP with no parameters its working accrately plzz send me the solution

tbAnswers.QuestionID,tbQuestionPaper.QuestionDescription,tbAnswers.YourAnswer,tbanswers.CorrectAnswerDes,tbansw

steve Great detail.

on March 17, 2008 at 11:48 pm | Reply

Is there ever a possibility of “name collision”, if the *same* user runs the same SP and two instances of the temp table are in existence at the same time? Stored proc is being executed from a web app, if that means anything. Thank you, Steve

Prashant Pandey

on March 18, 2008 at 4:29 pm | Reply

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Hi steve, its nice to hear that you like this article. “Is there ever a possibility of “name collision”, if the *same* user runs the same SP and two instances of the temp table are in existence at the same time? Stored proc is being executed from a web app, if that means anything.“ Now what i think is “yes there may be a possibility of name collision” i am not sure caz you are executing the SP from different web applications, but again the point is your sql server is same. So it may be arround 80% chances of name collision, But to avoid this what you can do is DROP the temporary table from your sp using the following code DROP TABLE #my_temp So there will be very minor possibilities of name collison. I’m discussing your topic further with some experts, so if will find anything special will let you know here. Thanks, keep visiting the blog.

steve

on March 18, 2008 at 4:46 pm | Reply

Thanks Prashant. Because of the way the web app is designed, it is improbable that the same user will run the same SP simultaneously, but because the SP will get relatively heavy use it is possible, so I appreciate your follow up. The #temp table however will have very few (less than 10 rows) so it’s lifetime should be quick and short.

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Thanks again, Steve

Prashant Pandey Thanks for your appreciation.

on March 18, 2008 at 5:00 pm | Reply

I said that there are chances of name collision because suppose if you have a sp which creates a temp table and use that, but remember its not deleting that table. So in query analyzer window if you execute this sp twice then in the 2nd time you’ll get an error something like “#temp_table already exists” So by considering this point in the case of web app, there may be a possibilities of name collision, because all the queries executing in the same SQL server. but as you said there are very few chances, but still there is 1% chance, So i thinks its better to drop temp tables if not used in any further query, because if u not delete these they will unnecessarily take the space in the SQL Server. Thanks,

km0ti0n Thank you for you detailed example.

on April 10, 2008 at 11:05 am | Reply

I need to take the example above one step further. For Example, if I make a temporary table I then want to innerJoin this temp table to another table to retunr rows form the joined table. This is my statment : DECLARE @THEIDS TABLE (CONTRACT_ID bigint)

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INSERT INTO @THEIDS (CONTRACT_ID) SELECT CONTRACT_ID FROM dbo.DOCUMENTS WHERE DOCUMENTS.DOCUMENT_ID > 100 GROUP BY CONTRACT_ID SELECT * FROM @THEIDS INNER JOIN CONTRACTS ON @THEIDS.CONTRACT_ID = CONTRACTS.CONTRACT_ID If I remove the end from “INNER JOIN” it works fine, but this errors with a error : “Must declare the variable ‘@THEIDS’.” Note the Group By Clause this is why I’ve had to create a temp table. Id it possible to create a temp table and then use it like this?

CHIN

on May 15, 2008 at 11:52 am | Reply

This is very usefull and clearfull article ever than others. thanks for it… CHIN.

vaibhav

on August 6, 2008 at 7:50 am | Reply

Its really good infromation abt tempory tables gives clear cut idea abt that Thanks lot!!!!!!!!!!!!!

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gyan shukla

on August 11, 2008 at 6:55 am | Reply

thanks alot……… these are great information regarding to temporary tables…and really help to understand DB query optimization

Praveen Krishna R Thank You, for this Detailed Guideline.

on September 19, 2008 at 2:52 pm | Reply

Rahul Wadekar Good Job…Thanks

on November 30, 2008 at 12:40 pm | Reply

Prabhat Dubey Hi,

on December 18, 2008 at 10:51 am | Reply

I have a query regarding the nested SP’s and use of Global Temp Table in it. The thing is I am calling SP2(lets say) from Sp1. This Sp2 is fetching multiple records from a table and these records needs to be sent back to SP1. Now the problem here is can I define a GTT in SP1 and insert data in it in SP2 and again retrieve it in SP1. If so can someone tell me how and what will be the syntax.. This is really a showstopper for me? Immediate response is highly appreciated.

Vishal open in browser customize

on January 22, 2009 at 3:50 pm | Reply
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Hi Prashant, You are doing a great job. The article is really very good. I have a question. I have a stored procedure in which at the end of the stored procedure after all the necessary code is written the global temporary tables are dropped. Once the stored procedure is executed how do i view the data in the temporary tables to troubleshoot the errors. I was told to comment on the code where the global temporary tables are dropped, I commented it but I am unable to save the stored procedure with the changes as it already exists and could not accomplish my goal. So how do i view the data generated in the temporary table. Any suggestions would be of great help. Thanks

Alexwebmaster Hello webmaster

on March 3, 2009 at 10:58 am | Reply

I would like to share with you a link to your site write me here [email protected]

How to Get Six Pack Fast

on April 15, 2009 at 3:11 pm | Reply

This is very up-to-date info. I’ll share it on Twitter.

tk Hi Prashant,

on June 17, 2009 at 1:45 pm | Reply

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Do you still check these comments? If so, here is my question re the need for a temp table/table variable. 1) I want to get a result set of job numbers, maybe 20 of them. 2) Then–for each of those jobs–I want to append each job into a parent table, then retrieve it’s @@IDENTITY. 3) Then–for each job–using it’s @@IDENTITY, I want to append child records for that job into another table. The way I’ve come up with is to retrieve my result set into a table variable, then a cursor to fetch each record, then do the appends for each record. I could write a loop on the client and call the sproc 20 separate times by passing the job, but that seems inefficient. Is there a better way to do this than the idea I had? Thanks!

Prashant Pandey Hi @tk,

on June 17, 2009 at 5:29 pm | Reply

I think you, first fetch the job numbers data and run a while loop on this job_number data and insert parent records. After that in the same loop fetch the PARENT ID from parent table on the basis of Job_number, and run another while loop on that data. if the job number is not present then you need to go with @@IDENTITY (but if the job number is not present I think you should check you’re database structure again for implementing proper relationships in between tables, so that fetching and insertion can be easy and faster)

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WHILE LOOP is there in SQL and they are better in all aspects than CURSORS Always prefer WHILE loops over CURSORS, both performs the same kind of functionality, whereas CURSORS are slow. Checkout this article: http://techahead.wordpress.com/2007/10/27/sqlcursors/ in this I have tried my best to explain WHY cursors are slow and how to use WHILE loop as an alternative for CURSORS 

swat

on July 6, 2009 at 12:22 pm | Reply

I have a stored proc A, which creates a temporary table. Inside this stored proc, I have another stored proc B where I am inserting the values into the temp table. After the completion of stored proc B, I am doing a select *from #temptable in proc A. Since I have created the temp table in proc A, I shouldnt have any problems while doing a select query. Am I correct here??

kalpesh

on July 2, 2010 at 2:48 am | Reply

we can use CTE(common table expression also insted of temp db……………………..

Muneer very usefull article

on December 14, 2010 at 8:01 am | Reply

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