Window functions in SQL Server

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In this video we will discuss window functions in SQL Server
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In SQL Server we have different categories of window functions
Aggregate functions - AVG, SUM, COUNT, MIN, MAX etc..
Ranking functions - RANK, DENSE_RANK, ROW_NUMBER etc..
Analytic functions - LEAD, LAG, FIRST_VALUE, LAST_VALUE etc...
OVER Clause defines the partitioning and ordering of a rows (i.e a window) for the above functions to operate on. Hence these functions are called window functions. The OVER clause accepts the following three arguments to define a window for these functions to operate on.
ORDER BY : Defines the logical order of the rows
PARTITION BY : Divides the query result set into partitions. The window function is applied to each partition separately.
ROWSor RANGE clause : Further limits the rows within the partition by specifying start and end points within the partition.
The default for ROWS or RANGE clause is
RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
Let us understand the use of ROWS or RANGE clause with an example.
Compute average salary and display it against every employee
We might think the following query would do the job.
SELECT Name, Gender, Salary,
AVG(Salary) OVER(ORDER BY Salary) AS Average
FROM Employees
As you can see from the result, the above query does not produce the overall salary average. It produces the average of the current row and the rows preceeding the current row. This is because, the default value of ROWS or RANGE clause (RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) is applied.
To fix this, provide an explicit value for ROWS or RANGE clause as shown below. ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING tells the window function to operate on the set of rows starting from the first row in the partition to the last row in the partition.
SELECT Name, Gender, Salary,
AVG(Salary) OVER(ORDER BY Salary ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS Average
FROM Employees
The same result can also be achieved by using RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
Well, what is the difference between ROWS and RANGE
We will discuss this in a later video
The following query can be used if you want to compute the average salary of
1. The current row
2. One row PRECEDING the current row and
3. One row FOLLOWING the current row
SELECT Name, Gender, Salary,
AVG(Salary) OVER(ORDER BY Salary ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS Average
FROM Employees
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After 20 years as software developer, it's the fist time that I pay attention to the adventage of window function, I'v been using the subqueries to get this kind of results, but with a very low performance in execution time.

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[@TheBaBaLand]

Great video! Thanks a lot! One thing I would add is that instead of writing BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING statement, we could remove the ORDER BY entirely and just write: AVG(SALARY) OVER() as Average

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[@venkatashasidharreddychali7485]

Hello Venkat gaaru, great work . Just to add on for the default value of ROWS or RANGE values in OVER() Clause, which varies based on ORDER BY clause --The default value of ROWS or RANGE clause is "RANGE BETWEEN UNBOUNDED PRECEEDING AND CURRENT ROW" If the ORDER BY clause is specified --The default value of ROWS or RANGE clause is "ROWS BETWEEN UNBOUNDED PRECEEDING AND UNBOUNDED FOLLOWING" If the ORDER BY clause is not specified i.e the entire window. Please correct me, if anything is wrong. Kudos to your work and Thank you🙏.

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Hello Justin - Thank you very much for the feedback. Means a lot. I have included all the sql tutorial videos, slides and text articles in sequence on the following page. Hope you will find it handy. www.pragimtech.com/courses/sql-server-tutorial-for-beginners/ When you have some time, can you please leave your rating and valuable feedback on the REVIEWS tab. It really helps us. Thank you. Good luck and all the very best with everything you are doing.

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[@frenkyb123]

if you use just this: AVG(Salary) OVER() - then whole set is used for computing average. But as soon you put order by into OVER, default value for ROWS and RANGE are taken into calculation. I would expect that just using ORDER BY should not affect calculation of AVG, but it does - it completely changes result set on which calculation is made.

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would test , thanks for letting us know

[@mermaidviv4517]

Yes, AVG(Salary) OVER () seems to have the same result

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very neat and to the point explanation !

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Awesome explanation

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Thanks for the video. It was really helpful

[@Csharp-video-tutorialsBlogspot]

+Dhanup Thank you for the feedback. This means a lot. Very pleased you found these videos useful. Dot Net & SQL Server training videos to help you become a web developer kzbin.infoplaylists?view=1&sort=dd Download videos for offline viewing www.pragimtech.com/Order.aspx Slides, Code Samples & Text Version of the videos can be found on my blog csharp-video-tutorials.blogspot.com Tips to effectively use our youtube channel kzbin.info/www/bejne/r2ibYYCtnb5qZtU To receive alerts, when new videos are uploaded, please subscribe to our channel kzbin.info If you like these videos, please click on the THUMBS UP button below the video May I request you to please share the link with your friends who you think would also benefit from them To teach is to learn Thanks Venkat

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This is super helpful. Thanks!

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Thank you Venkat Sir.

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Thank you so much and appreciate your work.

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Amazing ! No words !!!

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El Maestro returns

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Worth watching

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Thank you Venkat

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super helpful

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Very good explanation

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[@romiltube]

Hi Venkat, first off I enjoy your videos a lot, very helpful for interviews. I have a question related to analytic window function (sum) Could you please confirm which version of MS SQL Server are you using? Not 2008 right? Because, I tried running below query in 2008 version which is quite similar to what you demonstrated in of your slides in above video and it threw syntax error. select *, sum(sales) over(order by sales) as sumofsales from test1......error select *, sum(sales) over() as sumofsales from test1.......no error it ran successfully select *, sum(sales) over(partition by dealer) as sumofsales from test1.......no error it ran successfully The reason why the first query failed makes sense to me, because I believe there is no need at all for a order by argument in analytic window function; partition by argument might be required sometimes though (my 3rd query) I got confused because your query (quite similar to mine, as per above logic it should have failed since it contained order by arguement in analytical window function you demonstrated) ran successfully... Seems like you are using a different SQL version than mine...Just curious to know why your query did not fail :) Thanks again for comprehensive, well articulated and ofcourse free videos :) Regards, Romil

[@anshumanbupathi]

venkat sir could u explain realtime queries in fucntions

[@dengzhonghan5125]

Good tutorial. Partition by Gender order by Salary. Why salary is not in ascending order?

[@TheHimaBindu]

Very informative

[@mohammadal-aqrabawi7145]

Excellent!!!

[@f1r3shoot]

Отличное видео

[@cbdbee707]

Is there anyway to set the preceeding/following row clause programatically so that i can write one function that can output moving average of variable window width? (without using dynamic queries)

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5 star teacher

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great..

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Running average by accident, I love it!

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Thanks sir!!

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thanks so much

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Thanks

[@ishanvallabhkamat4254]

Why can't we directly use partition by instead of specifying Unbounded values? In video 108 this is achieved by Partitioning it right?

[@UrBoyiVignesh]

i think we have to use ID column for Order By Clause to avoid duplicates.....

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[@TheEverydayAnalyst]

If we just use OVER() without specifying anything inside over then it is an equivalent to mentioning UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING.

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