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What is window function in SQL with Examples? How and when to use them?

Hello folks, if you are wondering what is window function and what problem does it solve then you have to the right place. In this article, I would be explaining the windows functions with examples, how it works, and when you need to use it. Firstly what is a Window Function in SQL? A window function performs a calculation across a set of table rows that are similarly related to the current row. The current row is the row in which function evaluation occurs. This is comparable to the type of calculation that can be done with an aggregate function. But unlike regular aggregate functions like sum, avg, max, and min, that perform operations on an entire table.

The use of a window function does not cause rows to become grouped into a single output row. the rows retain their separate identities. Behind the scenes, the window function is able to access more than just the current row of the query result.

How to use Window Function in SQL?

Provided that there is a table called Products. The id , Product’s name, and Price columns are being selected as you can see. So the aggregate function AVG is called on the price column to give the new column called Average price and ordered by Product’s id.

A window function can also be called an analytic function, A window is a function that uses values from one or multiple rows to return a value for each row. (This contrasts with aggregate functions, which return a single value for multiple rows.)

Window functions have an OVER clause; any function without an OVER clause is not a window function, but it can only be an aggregate or single-row (scalar) function.


Let us say we have an employee table for instance.


 name               department       age       salary     

Grace Rolli       operations        21       150,000

 Cyndi Lo          operations        20       200,000

Wilms cole       maintenance      25      100,000

Alfred  Toni     maintenance       19       97,000

John Smith       sales                  23       50,000

 Chukwu Dan    sales                  27      75,000

So here is another practical code example. our employee table above, now we want to write a window function for it in SQL.

  1. SELECT name, age, department, salary,
  2. AVERAGE(Salary) OVER( PARTITION BY Department ORDER BY Age)
  3. AS Avg_Salary
  4. FROM employee;

Now, what this query is saying is that. name column, age column, department, and salary column should be selected from the employee table(check line 4). In line 2, an aggregate function "average" was performed on the salary column and after that has been done, what happens under the hood is that it creates another column as Avg_salary and puts each average salary of each employee in its appropriate rows.  

 The "Over" keyword which is at line two is what makes it a window function, now let us deal with that. You would see that inside the parenthesis of the "over" function there are lines of command to be carried out there. 

It is simply saying that function is partitioned by department and order by age. (There may be different kinds of things to do here, it all depends on what you what to do). 

What is window function in SQL with Examples? How and when to use them?
Fig 1.0 A visual representation of how window function works.


This partition keyword groups whatever you have specified into parts. In our case, department. All departments that are the same would be seen together. if we have 5 A's departments you would see them together you won't see any B's departments in between.  

After that now comes another set of similar departments. and so on and so forth, That is for partitioning." Order By" sorts it accordingly. and by default, it is in ascending order. so, After departments are partitioned then it begins to sort it according to each partitioned department. 

Below is what it looks like:

name               department       age       salary             Avg_Salary

Alfred  Toni     maintenance     19       90,000              45,000

Wilms cole       maintenance    25      100,000              50,000

Cyndi Lo          operations        20       200,000             100,0000

Grace Rolli       operations        21       150,000            75,000

John Smith       sales                 23       50,000               25,000

Chukwu Dan    sales                 27      70,000                35,000

Windows functions can access rows. It enables a sub-division or splitting into rows and They still maintain their different identities.


Types or Class of Windows functions

There are three classes or categories of windows Functions:

  • Ranking
  • Value
  • Aggregation

We shall be taking this one after the other in explanation

Ranking: The name “Ranking” is suggestive. When you hear ranking it means it has to do with the ranking or the top-leading information that is associated with the rows and columns. It displays the ranking information.

Value: Value gives you whatever that is assigned, which  correlates with each partition

Aggregation: It shows the aggregate values from numerical columns.

Conclusively, You may imagine, when do you need to use a window function? 

 A window function is used when you need to perform some calculations and you need the result of the calculation on each row.

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Thank you! for reading this article so far. If you have any questions feel free to ask in comments. 

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