Adventures in Machine Learning

Maximizing SQL Queries with NTILE() Function and Window Functions

NTILE() function and Window Functions in SQL

Have you ever wondered how to group your data into certain categories or quartiles and apply calculations such as maximum or minimum values within each grouping? Look no further than the NTILE() function and Window functions in SQL.

Window Functions and GROUP BY in SQL

When working with SQL, you may encounter an error message stating “Window Functions are Not Allowed in GROUP BY.” This error message is due to the order of operations and SQL’s method of evaluation. When grouping data using GROUP BY, the final result set needs to be produced before applying any window functions.

In other words, it’s not possible to group data by a window function. So, how do you use window functions in conjunction with GROUP BY?

One solution is to use a subquery or a common table expression. For instance, one can write a subquery that selects the data and applies the window function to column B.

Then, the main query can group the result set by column A and perform calculations on column B from the subquery. Another solution to utilize the power of window functions while grouping data is to calculate the quartiles.

For instance, using the NTILE() function, you can group the data into equal quartiles and perform calculations on each grouping. To accomplish this, the NTILE() function is applied to row number ordering by the numeric column of interest.

The output from NTILE() is then used to group data into equal quartiles.

NTILE() Function and Dividing Data into Quartiles

Let’s consider an example. Suppose you have a table of midterm results with a Student ID column and a Score column.

You’d like to group the data by quartiles and find the minimum, maximum, and count of students in each group. Here’s how it can be done using the NTILE() function:



MIN(Score) AS MinScore,

MAX(Score) AS MaxScore,

COUNT(StudentID) AS NumStudents

FROM MidtermResults

GROUP BY GradeQuarter


In the above SQL statement, we’re using the NTILE() function to divide scores into four quartiles (the 25th, 50th, and 75th percentile groups). The OVER clause specifies that the NTILE() function should be applied to the rows ordered by the Score column in descending order.

The result set is then grouped by the GradeQuarter column which represents the quartile groups and we’re calculating the minimum and maximum scores as well as the number of students in each quartile group. By using the NTILE() function, we can group the scores into equally sized quartiles and apply calculations on each group.

In this example, we’re able to compare the performance of students in each quartile group.


Overall, the NTILE() function and Window functions in SQL provide a powerful mechanism for grouping and analyzing data. When used with caution and proper syntax, they can help you to bring out valuable insights from your data.

Whether you’re partitioning your data by quartiles or carrying out more complex calculations, these functions can help you to process your data in a more meaningful way.

Importance of Understanding Window Functions

Window functions in SQL are among the most powerful and versatile features of the language. These functions can enable users to perform complex analytical operations, using just a few lines of code.

However, to harness the power of window functions effectively, it is vital to have an in-depth understanding of their syntax, common mistakes, and how they can be used in real-world scenarios.

Power of SQL Window Functions

The power of window functions lies in their ability to carry out calculations on a specified set of rows, called a window, within the result set of a query. They can be used to transform and aggregate data seamlessly, without having to create subqueries or joins.

Consider an example of a database table that contains sales data for a particular company. Suppose you wanted to calculate the running total of sales and the percentage contribution to total sales by region.

A simple query using the window function “SUM” can perform this calculation, as shown below.


SELECT region, sales,

SUM(sales) OVER (PARTITION BY region ORDER BY date) as running_total,

(sales / SUM(sales) OVER (PARTITION BY region)) as percent_of_total

FROM sales_data_table

ORDER BY date;


The query above demonstrates the power of window functions to apply complex calculations to large datasets easily. Here, the “SUM” function is applied to create a running total of sales for each region.

We are then dividing the sales for each row by the total sales per region to obtain the percent contribution for each region.

Syntax Details and Common Mistakes

It’s essential to understand the syntax and usage of SQL window functions to avoid common syntax errors. One common mistake is including the window function in the GROUP BY clause.

Remember that window functions are not valid in GROUP BY clauses. The window function operates after GROUP BY.

Another common mistake is forgetting to include the OVER() clause, which is necessary to specify the window boundary for the calculation.

Additionally, the partitioning clause can also be optimized to avoid the calculation of redundant partitions.

This can be achieved by specifying the PARTITION BY clause on already sorted data, reducing the time and resources required to calculate the window. Window Functions Course and Cheat Sheet

To harness the power of window functions, offers an excellent course dedicated to breaking down the syntax, usage, and tips on using window functions in SQL.

The course provides interactive exercises, quizzes, and short lessons to help users master the intricacies of SQL window functions. Past learners have praised the quality of the lessons and the depth of knowledge that they have gained in using window functions.

Additionally, provides a comprehensive cheat sheet on window functions, with detailed explanations of each function and the syntax needed to use them. The cheat sheet is a great reference tool for SQL programmers and analysts who want a quick refresher on window functions.


In conclusion, window functions in SQL can significantly enhance the analytical capabilities of SQL programmers and analysts. Mastering their syntax, common mistakes, and real-world scenarios can greatly improve the effectiveness and efficiency of SQL queries.

With’s courses and cheat sheets available, anyone can quickly master the art of using window functions in SQL. In conclusion, understanding SQL window functions is essential for any programmer looking to efficiently analyze large datasets.

Window functions can significantly enhance the capabilities of SQL queries, making complex calculations easier and more efficient. Syntax details and common mistakes are important to avoid for the successful execution of such queries. offers a comprehensive course on Window functions, along with a cheat sheet, which are invaluable to mastering these functions. Overall, with the rise of big data, utilizing SQL window functions can be a game-changer in terms of efficiency and can provide programmers with a competitive edge.

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