t-sql

ROW_NUMBER – How To Use It

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ROW_NUMBER is a function built-in to SQL Server that will return a row number for each record in your result set. You can further change resulting row number to reset the row number based on some value in the result set. I will show you examples of both below.

ROW_NUMBER Basics
To show the row number in SQL Server, you need to use the ROW_NUMBER function. This function is broken down in to two parts.

  • PARTITION BY – If you supply this parameter, then the row number will reset based on the value changing in the columns supplied. This is kinda like using a GROUP BY.
  • ORDER BY – This is the specified order of the row number that you would like. If you wanted the order of the row number to increment by an employee name (alphabetically), you do that here.

 
With this function I think examples will help paint the full picture.

In the following query, the results will show a row number with each record. The number will start at 1 and increment for every record in order of AnimalName. You can see that that the order of the row number increment is specified as a parameter in to the ROW_NUMBER() function.

SELECT	AnimalID,
	AnimalName,
	AnimalType,
	ROW_NUMBER() OVER(ORDER BY AnimalName) AS RowNumber
FROM	Animal

[table width=”500″ colwidth=”20|100|50″ colalign=”left|left|center|left|right”]
AnimalID,AnimalName,AnimalType,RowNumber
166,Alpaca,Mammal,1
168,Camel,Mammal,2
162,Carabao,Mammal,3
171,Cat,Mammal,4
163,Cattle,Mammal,5
184,Chicken,Bird,6
182,Deer,Mammal,7
185,Duck,Bird,8
186,Goose,Bird,9
189,Pigeon,Bird,10
188,Quail,Bird,11
187,Turkey,Bird,12
[/table]

 
In the above example, the row number never resets. It started at 1 and kept going through all the records. But what if you wanted to reset the row number back to 1 based on a value changing in your result set. In the following example, every time that the AnimalType changes we will reset the row number back to 1. This way, each AnimalType would have it’s own set of row numbers. We accomplish this by listing the columns that we want to group the row numbers by in the PARTITION BY parameter.

SELECT	AnimalID,
	AnimalName,
	AnimalType,
	ROW_NUMBER() OVER(PARTITION BY AnimalType ORDER BY AnimalName) AS RowNumber
FROM	Animal A
ORDER	BY AnimalType, AnimalName

[table width=”500″ colwidth=”20|100|50″ colalign=”left|left|center|left|right”]
AnimalID,AnimalName,AnimalType,RowNumber
184,Chicken,Bird[attr style=”background-color:#F2F5A9″],1[attr style=”background-color:#F2F5A9″]
185,Duck,Bird,2
186,Goose,Bird,3
189,Pigeon,Bird,4
188,Quail,Bird,5
187,Turkey,Bird,6
166,Alpaca,Mammal[attr style=”background-color:#F2F5A9″],1[attr style=”background-color:#F2F5A9″]
168,Camel,Mammal,2
162,Carabao,Mammal,3
171,Cat,Mammal,4
163,Cattle,Mammal,5
182,Deer,Mammal,6
[/table]

 
 
Good Things To Know
You can specify multiple columns in the PARTITION BY and ORDER BY parameters by separating them with a comma.

You can specify ASC or DESC in the ORDER BY parameter if you like as well.

 
Reference: http://msdn.microsoft.com/en-us/library/ms186734.aspx

MAXDOP – SQL Server’s Way To Avoid Parallelism

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SQL Server comes with built-in support for using parallelism for query execution. That means that SQL Server will split up the execution of a query among different CPUs if it thinks that it can get the query results faster that way. The default setting in SQL Server will use the number of CPU cores on the system as the maximum number of parallel processors to use. Okay… this is great, right?

 
The Grass Isn’t Always Greener
Although the thought of running 1 SQL query but using multiple processors sounds great, sometimes it can actually sometimes cause slower queries. A couple issues I have ran in to in our production environment are:

  • Sometimes the parallelism can take away CPU from other processes that are trying to run at the same time.
  • Sometimes the time that it takes to reassemble the results is longer than the not using parallelism. What this means is that when a query is split among different processors, it will need to merge them back together before it sends it to you. Sometimes this can be a longer task then just using 1 processor the whole time.

 
MAXDOP
MAXDOP is a query hint that can be added to the end of your SQL query to control the amount of parallelism that happens. When you use MAXDOP, you will specify the maximum number of parallelism threads that SQL Server uses for that query. In fact, the word MAXDOP stand for “Maximum Degree Of Parallelism”.

Here is an example of how to use the MAXDOP query hint to limit the number of parallel threads to 1 (essentially disabling the parallelism):

SELECT	*
FROM	Animal A
INNER	JOIN AnimalType AT ON AT.AnimalTypeID = A.AnimalTypeID
ORDER	BY A.AnimalName
OPTION	(MAXDOP 1)

To show how this affects the query, let me show you the execution plan when I supply the MAXDOP 1.
MAXDOP No Parallelism

Here is a screenshot of the execution plan of that exact same query without specifying the MAXDOP. You can see that it now uses parallelism.

MAXDOP Parallelism

 
Will Removing MAXDOP Help My Queries Run Faster
Is MAXDOP a silver bullet for making queries faster? No. In most cases you will just want SQL Server to manage the parallelism. This is usually the fastest/best way for your query to execute. But occasionally if you are having a slow query, you can try changing the MAXDOP setting. You will need to test your query before and after to see if there is significant improvement.

 
How Do You Know If Your Query Is Using Parallelism?
Here are a couple ways that I use to find out if parallelism is happening to a query.

  1. Check out the actual execution plan. After you run the query and the execution plan shows up, you will see a “Parallelism” step. You can see the screenshot above for what that looks like.
  2. Monitor the Activity Monitor in SQL Server Management Studio. This is a great way to detect if your production applications are causing parallelism. It’s really easy to see in Activity Monitor because it looks like the same line repeated many times.
    MAXDOP Activity Monitor

 
Reference: http://msdn.microsoft.com/en-us/library/ms181714.aspx

Sql Server Version

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Here is a super simple query that will get you your current SQL Server version. Enjoy!

SELECT	'SQL Server ' +
		CASE	CAST(SUBSTRING(CAST(SERVERPROPERTY('ProductVersion') AS VARCHAR(100)), 1, CHARINDEX('.', CAST(SERVERPROPERTY('ProductVersion') AS VARCHAR(100)))-1) AS INT)
				WHEN 8 THEN '2000'
				WHEN 9 THEN '2005'
				WHEN 10 THEN '2008'
				WHEN 11 THEN '2012'
				WHEN 12 THEN '2014'
		END	+ ' ' +
		CASE	CAST(SERVERPROPERTY('ProductLevel') AS VARCHAR(100))
				WHEN 'RTM' THEN ''
				ELSE CAST(SERVERPROPERTY('ProductLevel') AS VARCHAR(100)) + ' '
		END +
		CAST(SERVERPROPERTY('Edition') AS VARCHAR(100)) AS SQLServerVersion

SQL Server Rename Column

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This article will cover how to rename a column in SQL Server. The function of a SQL Server rename column is not performed too often. Usually we do this when we roll out a new product and we want to make a column more generic.

 
SQL Server Rename Column (SQL Server 2005 and beyond)

They make this super simple. Just remember to put to single quotes around the parameters.

EXEC	sp_rename 'Animals.AnimalName', 'AnimalRealName', 'COLUMN'

The above SQL Server rename column code snippet has a few parts to it. Here is the breakdown:

  • sp_rename – This is a system stored procedure will rename different types of objects in SQL Server.
  • ‘Animals.AnimalName’ – This is the tablename and column that I would like to rename.
  • ‘AnimalRealName’ – This is the new name that I would like to call the column.
  • ‘COLUMN’ – This is the type of object that you would like to rename is. To rename a column, always leave this value ‘COLUMN’.

 
If you would like to know how to do a SQL Server rename column for SQL Server versions prior to 2005, please leave a note in the comments.

 
 
Reference: http://msdn.microsoft.com/en-us/library/ms188351(v=sql.90).aspx

SQL Server 2014 Feature – Non-Clustered Indexes For Table Variables

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This is truly an amazing feature to add to SQL Server 2014. I use table variables all the time and this was the only thing I didn’t like about them. Up to this point, SQL Server did not support having non-clustered indexes on table variables (the one with the name that starts with the @). If you wanted to do this, you had to create/use a temp table (the one with the name that starts with #).

With SQL Server 2014, they changed this and now allow the table variables to have non-clustered indexes as well. To do this, we simply add a little bit of extra code after the column declaration.

DECLARE	@AnimalTableVar	TABLE
( AnimalID		INT,
  AnimalName	VARCHAR(50)	INDEX IX_AnimalTempTable_AnimalName
)

In the sample above, we create a table variable called @AnimalTableVar. Then when we declare the column AnimalName, we add an index called IX_AnimalTempTable_AnimalName.

Here is how selecting from this table shows up in the execution plan!
Non-Clustered Indexes For Table Variables

Reference: http://msdn.microsoft.com/en-us/library/ms188927(v=sql.120).aspx