ROW_NUMBER – How To Use It


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.

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.

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

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.

AnimalID AnimalName AnimalType RowNumber
184 Chicken Bird 1
185 Duck Bird 2
186 Goose Bird 3
189 Pigeon Bird 4
188 Quail Bird 5
187 Turkey Bird 6
166 Alpaca Mammal 1
168 Camel Mammal 2
162 Carabao Mammal 3
171 Cat Mammal 4
163 Cattle Mammal 5
182 Deer Mammal 6

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.


Varchar Vs Nvarchar


People often want to know the difference between varchar vs nvarchar. If you are new to nvarchar, I don’t blame you for not understanding the difference… as they are very similar.

The major difference is that nvarchar stores data as Unicode, where varchar does not. This uses more space, but provides flexibility to handle foreign characters that would not fit in a varchar field.

How Varchar And Nvarchar Are Similar

  • They are both used to store text/string data in them
  • The amount of space that both use depends on the size of the data that you are putting in it. It grows with every character that you store.

How Varchar And Nvarchar Are Different

  • Nvarchar stores data as Unicode. Varchar stores data as non-Unicode.
  • Varchar stores data at 1 byte per character. Nvarchar stores data at 2 bytes per character.
  • Varchar supports up to 8000 characters in the field definition. Nvarchar only supports up to 4000 characters. Varchar(max) and nvarchar(max) not included.

My Thoughts On Which One To Use
Where I am at, I deal with a lot of data. Although it would be nice to be able to support all foreign characters in the database, it is likely that I will never get it. My philosophy is that unless you need to support these extended Unicode characters, just make the field varchar. Nvarchar uses double the space as varchar. Unless your business has a requirement for this, just save the space.

SQL Server Zero Pad Left


In a recent project, I had the task of padding a number with zeros. SQL Server doesn’t have a built in Zero Pad Left function. SQL Server does have a Right function. You can use this to create a zero padded left string.

In the code above, I am take 10 zeros, appending a text value to it, then taking the right 10 characters. The end result is the same as padding the left with zeros.

SQL Server Zero Pad Left Function
You can take this a step further and create a scalar function out of it.

MAXDOP – SQL Server’s Way To Avoid Parallelism


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 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):

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


Sql Server Version


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