9.4.08

T-SQL Challenges - Part 3 - Temporary tables

Digging into the logics behind Temporary tables in sql server:
I have noticed many interesting articles on temporary table in sql server.
But, this article provides some unknown facts on sql server.
Assuring you an interesting fact on sql server temporary table.
Know about Temporary table:
Temporary tables are special tables created in temp database. It has a prefix '#' for local temporary table
or "##" for global temporary table.
The temporary table will get deleted if the session is expired or the server restarts.
Local temporary table: This can be used with in a specific object and cannot be
shared with other system objects. Each table is associated with a specific session id
and we cannot use it in another session.
For ex: I am creating a temporary table A in the stored procedure SP1, If i try to
use the same temporary table in another stored procedure SP2. It won't work because
its not in this specific scope. To recover this problem, we have to Global temporary table.
Global temporary table: Its nothing but a global table accessed by different
system objects.
Ok, am stopping all those basics and lets enter into some R&D work,
I had a situation to construct a temporary table using dynamic SQL. OOPS!! am finding some
strange problem to achieve it. Let's see the code,
Code Snippet
declare @string varchar(1000)
set @string = 'SELECT * INTO #TempTable FROM dbo.SAMPLE'
exec (@string)
SELECT * from #TempTable

I am getting the following error,
"Invalid object name '#TempTable'."
I wondered to see this message and started checking the syntax, but everything is
ok and i don't find any way to fix this problem.
I've tried with ordinary table,
Code Snippet
declare @string varchar(1000)
set @string = 'SELECT * INTO TempTable FROM dbo.SAMPLE'
exec (@string)
SELECT * from TempTable
OOPS, its working fine.
But my business functionality forces me to create a temporary table. I don't find
any other way to go,
Code Snippet
declare @string varchar(1000)
create table #TempTable(id int)
set @string = 'insert INTO #TempTable select id FROM dbo.SAMPLE'
exec (@string)
SELECT * from #TempTable
Atlast, i got some other way to achieve the same. But, i dont know what's the exact problem lies behind.
Digging towards it, i found some other way.
I resolved the problem by using global temporary table instead of local temporary table
It's working fine.
Code Snippet
declare @string varchar(1000)
set @string = 'SELECT * INTO ##TempTable FROM dbo.SAMPLE'
exec (@string)
SELECT * from ##TempTable
Solution statement:
After a thorough search, i found that the problem lies with the exec statement.
Exec statement will try to create a seperate session and my local temporary table
won't lie in this newly created session.
Interesting right!!!!!
Happy learning.
Regards,
Venkatesan Prabu . J

2 comments:

Anonymous said...

I assume, like me, you are using a temporary table so that more than one user can run the process without trying to create/access the same data. Problem is that a global temporary table is visible to everyone and, if someone instigates the process before a previous instigation is complete, they will get a "table already exists error".

Venkatesan Prabu @ HCL said...

Its no the case...
You will get seperate tables based on the session id maintained by the user


Thanks
Venkat

My T-SQL Gallery @code.msdn.microsoft


Created my own T-SQL Gallery in Microsoft site. Do visit the same and share your feedback,

http://code.msdn.microsoft.com/VenkatSQLSample/Thread/List.aspx

Thanks and Regards,
Venkatesan Prabu .J

SQL Server Interview questions - Part 1

What is the significance of NULL value and why should we avoid permitting null values?
Null means no entry has been made. It implies that the value is either unknown or undefined.We should avoid permitting null values because Column with NULL values can't have PRIMARY KEY constraints. Certain calculations can be inaccurate if NULL columns are involved.

What is SQL whats its uses and its component ?
The Structured Query Language (SQL) is foundation for all relational database systems. Most of the large-scale databases use the SQL to define all user and administrator interactions. It enable us to retrieve the data from based on our exact requirement. We will be given a flexibility to store the data in our own format.


The DML component of SQL comprises four basic statements:
* SELECT to get rows from tables
* UPDATE to update the rows of tables
* DELETE to remove rows from tables
* INSERT to add new rows to tables


What is DTS in SQL Server ?
Data Transformation Services is used to transfer the data from one source to our required destination. Considering am having some data in sql server and I need to transfer the data to Excel destination. Its highly possible with dialogue based tool called Data Transformation services. More customization can be achieved using SSIS. A specialized tool used to do such migration works.


What is the difference between SQL and Pl/Sql ?

Straight forward. SQL is a single statement to finish up our work.Considering, I need some data from a particular table. “Select * from table” will fetch the necessary information. Where as I need to do some row by row processing. In that case, we need to go for Procedural Logic / SQL.

What is the significance of NULL value and why should we avoid permitting null values?
Null means no entry has been made. It implies that the value is either unknown or undefined.We should avoid permitting null values because Column with NULL values can't have PRIMARY KEY constraints. Certain calculations can be inaccurate if NULL columns are involved.

Difference between primary key and Unique key?
Both constraints will share a common property called uniqueness. The data in the column should be unique. The basic difference is,
· Primary key won’t allow null value. Whereas, unique key will accept null value but only one null value.
· On creating primary key, it will automatically format the data inturn creates clustered index on the table. Whereas, this characteristics is not associated with unique key.
· Only one primary key can be created for the table. Any number of Unique key can be created for the table.

Select Statement in SQL Server

Select Statement in SQL Server

String Functions in sql server

String Functions in sql server
Substring/Len/replace/Ltrim/Rtrim

SQL Server Interview Question - Part 2

What is normalization?

Normalization is the basic concept used in designing a database. Its nothing but, an advise given to the database to have minimal repetition of data, highly structured, highly secured, easy to retrieve. In high level definition, the Process of organizing data into tables is referred to as normalization.


What is a stored procedure:
Stored procedures are precompiled T-SQL statements combined to perform a single task of several tasks. Its basically like a Macro so when you invoke the Stored procedure, you actually run a set of statements. As, its precompiled statement, execution of Stored procedure is compatatively high when compared to an ordinary T-SQL statement.


What is the difference between UNION ALL Statement and UNION ?
The main difference between UNION ALL statement and UNION is UNION All statement is much faster than UNION,the reason behind this is that because UNION ALL statement does not look for duplicate rows, but on the other hand UNION statement does look for duplicate rows, whether or not they exist.

Example for Stored Procedure?
They are three kinds of stored procedures,1.System stored procedure – Start with sp_2. User defined stored procedure – SP created by the user.3. Extended stored procedure – SP used to invoke a process in the external systems.Example for system stored proceduresp_helpdb - Database and its propertiessp_who2 – Gives details about the current user connected to your system. sp_renamedb – Enable you to rename your database


What is a trigger?

Triggers are precompiled statements similar to Stored Procedure. It will automatically invoke for a particular operation. Triggers are basically used to implement business rules.


What is a view?
If we have several tables in a db and we want to view only specific columns from specific tables we can go for views. It would also suffice the needs of security some times allowing specfic users to see only specific columns based on the permission that we can configure on the view. Views also reduce the effort that is required for writing queries to access specific columns every time.


What is an Index?
When queries are run against a db, an index on that db basically helps in the way the data is sorted to process the query for faster and data retrievals are much faster when we have an index.


What are the types of indexes available with SQL Server?

There are basically two types of indexes that we use with the SQL ServerClustered -

1. It will format the entire table, inturn physically sort the table.

2. Only one clustered index can be created for a table.

3. Data will be located in the leaf level.

4. By default, primary key will create clustered index on the table.

Non-Clustered Index

1. It wont touch the structure of the table.

2. It forms an index table as reference to the exact data.

3. A reference to the data will be located in the leaf level.

4. For a table, we can create 249 non clustered index.

Happy Learning!!!
Regards,
Venkatesan Prabu .J

SQL Interview question

Extent Vs Page?

Pages are low level unit to store the exact data in sql server. Basically, the data will be stored in the mdf, ldf, ndf files. Inturn, pages are logical units available in sql server.The size of the page is 8KB.

Eight consecutive pages will form an extent 8 * 8KB = 64KB.

Thus I/O level operation will be happening at pages level.The pages will hold a template information at the start of each page (header of the page).

They are,

1. page number,

2. page type,

3. the amount of free space on the page,

4. the allocation unit ID of the object that owns the page.

Extents will be classifed into two types,

1. Uniform extents

2. Mixed extents

Uniform Extents:It occupied or used by a single object. Inturn, a single object will hold the entire 8 pages.Mixed

Extents:Mulitple objects will use the same extent. SQL Server will allow a max of eight objects to use a shared extent.

Property of SQL Server :Initally if an object is created, sql server will allocate the object to the mixed extent and once if the size reaches 8 pages and more... immediately, a new uniform extent will be provided for that particular object.

Herecomes, our fragmentation and reindexing concepts.

Best Joke - Enjoy it

Best Joke - Enjoy it