Window Functions

Jill Thornhill

SQL LAG() Function

The LAG() function – one of SQL’s window functions – is an important tool for planning and trend analysis. In this article, I’ll demonstrate how to include SQL LAG() in your queries using a few real-world examples. SQL window functions, also known as analytical functions or OVER functions, add a new dimension to data analysis. They allow you to include aggregates or data from other rows alongside the current row.

Tihomir Babic

SQL Window Functions Guide

Every data analyst can benefit from knowing SQL window functions. They up your data analysis game and open doors to analyzing data on an entirely new level. This article is your starting point for the world of SQL window functions.   SQL window functions allow you to show all the data rows and their aggregate values at the same time. Sounds a bit like GROUP BY on steroids, doesn’t it?

Agnieszka Kozubek-Krycuń

SQL FIRST_VALUE Function

Discover the power of the FIRST_VALUE function, a window function that returns the first value in an ordered partition of the result set. This function is useful for anyone doing data analysis in SQL. Window functions are a powerful category of functions that enable doing sophisticated data analysis in SQL. These functions operate over a "window" of rows related to the current row, allowing for preparing complex reports like running totals, moving averages, preparing rankings, and others.

Agnieszka Kozubek-Krycuń

SQL LEAD Function

Get to know the SQL LEAD function, an important function to know for anyone working with SQL in data analysis. Learn through examples how to use this function in practice. The SQL LEAD function is a very important and useful SQL window function. SQL window functions are essential for doing efficient data analysis. They allow you to work with groups of rows and single rows at the same time. They make writing complex reports easier.

Jakub Romanowski

Free Course of the Month: PostgreSQL Window Functions

Are you looking for the best way to master SQL window functions? Throughout March 2024, you can access the interactive course Window Functions in PostgreSQL for free. Boost your skills at no cost. Hurry, time is running out! Need a free PostgreSQL course on window functions? You've come to the right place! If you are reading this article, you probably know PostgreSQL is one of the most popular databases in the world.

Tihomir Babic

What Are SQL Window Functions?

Need to up your data analysis game? Learn these SQL window functions here and you’ll take your analysis skills to the next level. Practice exercises and detailed explanations included! When I first heard about SQL window functions, I thought it was some strange marriage between SQL and Windows. I was wrong. The SQL window functions have nothing to do with that famous operating system. They are SQL functions that do computations on the set of rows related to the current row.

Jill Thornhill

BigQuery Window Functions Explained

Keeping up with data analysis trends gives your organization – and your CV – the cutting edge. In this article, we'll look at BigQuery window functions and how you can use them to gain deeper insights into your data. Google's BigQuery, launched in 2010, is gaining traction as a popular choice with organizations needing to analyze large quantities of information quickly and to compare their own data against statistical data in the public domain.

Ignacio L. Bisso

How to Use ROW_NUMBER OVER() in SQL to Rank Data

Sometimes you need to know the position of rows in a result set. Learn how using ROW_NUMBER and OVER in SQL can make it happen! Have you ever needed to add a sequential number to the records returned by an SQL query? Or perhaps you need to create a ‘top n’ report based on a specific ranking. In any of these cases, you need to calculate the position of the row in the ranking.

Martyna Sławińska

11 SQL Window Functions Exercises with Solutions

In this article, we present 11 practice exercises involving SQL window functions, along with solutions and detailed explanations. SQL window functions are a powerful feature that lets us extract meaningful insights from our data easily, yet few SQL courses offer SQL window functions exercises. This makes practicing window functions quite difficult. In this article, we’ll give you 11 practice exercises that feature window functions. All exercises shown in this article come from our interactive courses Window Functions and Window Functions Practice Set.

Jakub Romanowski

Free Course of the Month – Window Functions Practice Set

Do you want to enhance your SQL skills and learn how to prepare complex SQL reports? Maybe you've learned SQL window functions but need a refresher. This month, our unique SQL course Window Functions Practice Set is completely free! You're probably wondering ‘What's the catch?’. Why do we give something for free to our users? Well... we want to support people like you who are constantly striving to be better.

Dominika Florczykowska

An Overview of MySQL Window Functions

MySQL window functions are very helpful when you want to create meaningful reports with SQL. In this article, we’ll demonstrate the most common window functions in MySQL and explain how to use them. MySQL 8.0 introduced a new feature: window functions. These functions are very useful to data analysts and anyone who creates reports using MySQL. Using them, you can easily compute moving averages, cumulative sums, and other calculations over specified subsets of your data.

Martyna Sławińska

How to Use SUM() with OVER(PARTITION BY) in SQL

Discover real-world use cases of the SUM() function with OVER(PARTITION BY) clause. Learn the syntax and check out 5 different examples. We use SQL window functions to perform operations on groups of data. These operations include the mathematical functions SUM(), COUNT(), AVG(), and more. Typically, you use SUM() function with GROUP BY. In this article, we will explain what SUM() with OVER(PARTITION BY) does in SQL. We’ll show you the most common use cases in real-world applications to determine the ratio of the individual row value to the total value, calculate running totals, and find a custom order total that includes discounts for certain products.

Tihomir Babic

How to Practice SQL Window Functions Online

Learn and practice SQL window functions from the comfort of your home! Here’s how to do it. The last few years have shown that working from home has many benefits over the daily  commute. No wonder people are reluctant to go back to the office! They have much more time while doing the same work. What do you do with the extra time? Whatever you want. It’s your time!

Tihomir Babic

What Are Window Functions in SQL?

In this article, we discuss what window functions are and how they help you do your job as a data analyst or specialist. No, window functions have nothing to do with the operating system. The window here refers to a set of rows over which the function aggregates data. To learn how window functions work, what functions there are, and how to apply them to real-world problems, it’s best to take the Window Functions course.

Ignacio L. Bisso

How to Use the SQL PARTITION BY With OVER

At the heart of every window function call is an OVER clause that defines how the windows of the records are built. Within the OVER clause, there may be an optional PARTITION BY subclause that defines the criteria for identifying which records to include in each window. Read on and take an important step in growing your SQL skills! What Is the PARTITION BY Clause in SQL? The SQL PARTITION BY expression is a subclause of the OVER clause, which is used in almost all invocations of window functions like AVG(), MAX(), and RANK().

Jakub Romanowski

SQL Course of the Month – Window Functions in PostgreSQL

This is what distinguishes ordinary SQL users from professionals – SQL window functions! Would you like to practice them in PostgreSQL, on one of the best interactive SQL courses in the world, and pay nothing for it? Throughout November, you have access to the Window Functions in PostgreSQL course for FREE. You read it right: you won't pay a penny! What's the catch? Why would someone give you a free course?

Ignacio L. Bisso

What Is a Rolling Average and How Do You Compute It in SQL?

In this article, we will show three different examples of using SQL to calculate rolling averages. Learn this and you will take an important step in your SQL growth! A rolling average is a metric that allows us to find trends that would otherwise be hard to detect. It is usually based on time series data. In SQL, we calculate rolling averages using window functions. First, let’s talk about what rolling averages are and why they’re useful.

Ignacio L. Bisso

What Is the OVER() Clause in SQL?

Window functions are one of SQL’s most powerful resources, but they are not frequently used by the average SQL developer. In this article, we will explain how you can define different kinds of window frames using the OVER clause. The OVER clause is essential to SQL window functions. Like aggregation functions, window functions perform calculations based on a set of records – e.g. finding the average salary across a group of employees.

Tihomir Babic

Difference Between ROWS and RANGE in Window Functions

Are you familiar with ROWS and RANGE window functions but not sure how they are different? This article makes those differences clear. In using SQL window functions, you’ve probably come across ROWS and RANGE clauses. If you haven’t, they are both parts of the Window Functions course. If you have, it’s quite likely you have been left confused why there are two keywords for the same function. Well, they are not the same!

Kateryna Koidan

5 Practical Examples of Using ROWS BETWEEN in SQL

SQL window functions are tremendously useful for calculating complex aggregations like moving averages or running totals. The ROWS clause allows you to specify rows for your calculations, enabling even more sophisticated window frames. Here are five practical examples of leveraging the ROWS BETWEEN clause in SQL. Window functions (also called OVER functions) compute their result based on a sliding window frame (i.e. a set of rows). They are similar to aggregate functions in that you can calculate the average, total, or minimum/maximum value across a group of rows.

Tihomir Babic

How to Find the Nth-Highest Salary by Department with SQL

Learn how to find the nth-highest salary in SQL, and you’ll learn how to get the nth value in any data. In this article, I’ll focus on one problem and give you several solutions to that problem. The problem is stated in the title already: find the nth-highest salary by department using SQL. This may sound too specific. But by learning how to solve this, you’ll be able to find the nth value in any data, not just salaries.

Ignacio L. Bisso

How to Define a Window Frame in SQL Window Functions

The window function is a super powerful resource of the SQL language. At the core of any window function, there is a set of records, called the window frame, defined using an OVER clause. Knowing what records are in the window frame, how they are ordered, and what their upper and lower bounds are, are critical in understanding how window functions work. In this article, we will analyze and explain with examples how you can define different types of window frames.

Ignacio L. Bisso

Top 10 SQL Window Functions Interview Questions

Many interesting job positions require SQL skills – and that includes window functions, which are not commonly taught in online courses. In this article, I will cover the top window function questions for every experience level. If you’re going for a job interview for an advanced SQL position or for intermediate to advanced data analyst positions, you’ll probably be asked about your knowledge of SQL window functions. Don’t panic! Although these functions aren’t commonly covered in online courses, we’ve got the answers right here.

Tihomir Babic

RANGE Clause in Window Functions: 5 Practical Examples

What is a RANGE clause in SQL window functions? Five practical examples will show you how and when to use it. The RANGE clause is used quite rarely in SQL window functions. I don't know why; maybe people are not used to it. This is a shame, because it's far from a pointless clause; it can be very useful, and I'll show you that in five examples. To learn how window functions work, what functions there are, and how to apply them to real-world problems, it’s best to take the Window Functions course.

Tihomir Babic

Business SQL: How to Calculate Revenue Growth in SQL

Can you use SQL in business? Of course you can! For example, you can calculate revenue growth in SQL – here’s how. As someone who works with data, you already know that SQL is the most used tool in data analysis. But how is SQL helpful in business use? Well, the answer is obvious: analyze business data using SQL! However, there are some specifics with business data that can be tricky if you’re not used to it.

Ignacio L. Bisso

How to Compute Year-Over-Year Differences in SQL

How did your business do this year compared to last year? The year before? Find out how to use SQL to calculate year to year and month to month differences. Year-over-year (YOY) comparisons are a popular and effective way to evaluate the performance of several kinds of organizations. Year to year differences are easy to understand – e.g. it’s easy to understand that your revenue growth is 35% year to year.

Tihomir Babic

8 Best SQL Window Function Articles

Do you need a starting point for learning SQL window functions? Not sure which articles provide a solid foundation? We dove into the internet rabbit hole and found the eight best articles dealing with window and analytic functions. There are plenty of articles dealing with SQL window functions. Type those words into Google and you’ll find a huge number of articles returned. Reading articles is a good start if you want to learn something about window or analytic functions in SQL – provided, of course, that the articles you choose are trustworthy and up to date.

Tihomir Babic

What Is Advanced SQL?

Updated on: May 30, 2024 Are you confused about advanced SQL skills? What are they? This article will explain what advanced SQL can mean, especially as we use it on LearnSQL.com. I’m sure you find the phrases ‘advanced SQL skills’ or ‘advanced SQL topics’ very often. You read one article about advanced SQL and you’re happy with how easy these advanced topics seem to be. Then you talk to someone and you see they consider everything you know as basic SQL knowledge.

Tihomir Babic

How to Find the Next Non-NULL Value in SQL

You need to find the next non-NULL value in a time series, but you don’t know how. Can you even do that in SQL? Yes, you can! This article will show you what to do. If you work with SQL, you will sooner or later confront NULL values. Having NULLs in a database is almost unavoidable. However, sometimes you want to avoid them in your reports. This is quite often true when you’re analyzing time series data; NULL values mean there’s no data available.

Kateryna Koidan

What Is the RANK() Function in SQL, and How Do You Use It?

Who are your top 10 customers? Who are the top-performing employees in each department? To answer questions like these, you’ll need to rank the output of your SQL query. Let’s see how the RANK() function assists with ranking results in SQL.   There are many use cases in which you might need to rank rows in SQL. You may want to rank customers based on the sales volume, students based on their exam results, company assets based on their present value, departments based on the number of employees, or users based on their date of registration.

Agnieszka Kozubek-Krycuń

Why Aren’t SQL Window Functions Allowed in GROUP BY?

It’s true – the SQL order of operations means window functions aren’t allowed in GROUP BY. But you can use them anyway with this special trick. SQL window functions are one of the language’s most powerful features. However, the syntax of window functions is not easy to master. It has lots of details that can cause beginners to stumble. One of the most common traps is trying to use window functions in GROUP BY.

Zahin Rahman

Why Learn SQL Window Functions in 2021?

SQL window functions provide some extremely powerful and convenient features for data practitioners. This article explains why you should learn window functions and when you should use them. I’ll also provide some examples to help you visualize the concepts. Window functions are part of advanced modern SQL – knowledge of which will definitely enrich the repertoire of any SQL user. Window functions have a broad range of uses and can be applied to almost all facets of business.

Marija Ilic

SQL Window Functions for Managers: Who Gets a Raise?

Window functions are one of the most powerful features in modern SQL. In this article, you will learn why window functions are so great. I will list you the most useful window functions and briefly explain when and how you can use them. Simple SELECT statements, in combination with WHERE, GROUP BY, and HAVING, are sufficient for many analyses at work. I used standard SQL for many years before I realized there is something more.

Zahin Rahman

Best Online Courses for SQL Window Functions

In this article, I will summarize the top online tutorials and courses available on SQL window functions for enthusiasts of data analysis, data science, and data engineering. The information is here to help you embrace the topic with ease and feel comfortable using window functions in SQL databases. SQL window functions can help you solve some of the most common analysis challenges in business: What are the N most popular products for each month of the year?

Kateryna Koidan

Aggregate Functions vs Window Functions: A Comparison

If you aren’t familiar with SQL’s window functions, you may wonder how they differ from aggregate functions. When should you use window functions? In this article, we’ll review window functions and aggregate functions, examine their similarities and differences, and see which one to choose depending on what you need to do. After you’ve tackled basic SQL, you’ll probably want to get into some of its more advanced functions. That’s great; these functions make reporting and analysis easier.

Martyna Sławińska

A Comprehensive Introduction to Window Functions in MySQL

Do a deep dive into the working principles, syntax, and applications of various MySQL window functions. There are many open source databases you could choose; MySQL is consistently one of the most popular. But until 2018, there was no provision for including window functions in MySQL. Fortunately, all that has changed – starting from MySQL 8.0, SQL window functions were now available to MySQL users. Please note that window functions are available only in MySQL 8.

Tihomir Babic

Six Window Function Templates to Save in Your Code Editor

Do you use window functions often enough to know them well but not often enough to remember all the details? Here are six window function codes you should save as templates. Window functions are a handy SQL option, but it doesn’t mean you use them all the time. Maybe you only use them occasionally; if that’s the case, you probably find yourself needing to refresh your memory every time you use them.

Ignacio L. Bisso

What Is the MySQL OVER Clause?

If you want to learn window functions in MySQL, you need to understand the OVER clause. In this article, we use real-world examples to explain what the MySQL OVER clause is, how it works, and why it’s so awesome. In 2018, MySQL introduced a new feature: window functions, which are accessed via the OVER clause. Window functions are a super powerful resource available in almost all SQL databases. They perform a specific calculation (e.

Dorota Wdzięczna

Six Examples Using MySQL Window Functions

Window functions are an advanced SQL feature available in most popular databases. MySQL had not supported them for a long time, but that changed in Version 8.0. They are helpful not only for analysts and people who create reports, but also for other professionals who use databases to select data needed. In this article, we explain the syntax of some popular window functions with practical examples. What Is a Window Function?

Marija Ilic

What a Moving Average Is and How to Compute it in SQL

Want to dive deep into time series data and examine long-term trends? Want to know what a moving average is and how to compute it in SQL? Then this article is for you. I’ll explain a powerful feature in SQL called window functions and show how you can calculate moving averages using them. The best way to learn about SQL window functions is the interactive Window Functions course at LearnSQL.

Tihomir Babic

How to Calculate the Length of a Series with SQL

What is a time series and why bother to calculate its length using SQL? How do we calculate the length of a series in SQL? I can answer that in two words: window functions! Yes, you’ll need some knowledge of SQL window functions to calculate series length. But what is a time series, and why would you want to find out how long it is? What Is a Series?

Tihomir Babic

Who Should Learn SQL Window Functions?

Do you want to learn how SQL window functions can help you at your job? This article will show you examples from various business applications where they can be very useful. I won’t be explaining what SQL window functions are in this article, but rather how to use them. If you’re not familiar with window functions or their syntax, don’t worry. Here’s an article that can help you with an introduction to SQL window functions.

Marija Ilic

How to Analyze a Time Series in SQL

Values ordered by time are called a time series. In this article, I’ll show you the most frequent patterns and teach you how to write queries for time series in SQL with the help of window functions. Maybe you’ve had the opportunity to analyze some variables where each value was associated with a time value. Such data – where values are ordered by time – is called time series data.

Dorota Wdzięczna

The LAG Function and the LEAD Function in SQL

LAG() and LEAD() are positional functions. A positional function is a type of window function. If you are not familiar with when and how to use them, what the syntax of each function is, why you might want to use them, and what the differences are, read on! LAG() and LEAD() are positional functions. These are window functions and are very useful in creating reports, because they can refer to data from rows above or below the current row.

Ignacio L. Bisso

How to Calculate the Difference Between Two Rows in SQL

Calculating the difference between two rows in SQL can be a challenging task. It is possible – and there’s more than one way to do it. In this article, I’ll explain how to use the SQL window functions LEAD() and LAG() to find the difference between two rows in the same table. The best way to learn window functions is our interactive Window Functions course. It contains over 200 hands-on exercise that will teach you all window functions concepts, from simple OVER() clause to complex topics such as combining window functions and GROUP BY clause.

Tihomir Babic

How to Practice SQL Window Functions at Home

Are you looking for a way to maintain your knowledge of SQL window functions? What’s the best way to practice them at home? Here are some suggestions. Learning something new feels great, but it’s also arguably the easiest part of gaining knowledge. You’ve probably experienced that yourself. You struggle with learning an idea, and then you finally get it! There’s the feeling you’ve learned something. And then, very soon, you practically have to start all over again with the very same concepts.

Tihomir Babic

5 Advanced SQL Constructions Every Data Analyst Should Learn

Does your job involve data analysis? Here are five examples of how advanced SQL can help you in your daily work. I’m a data analyst and I have to say – data analysts can be quite strange. Other people are usually afraid of immense amounts of data; we enjoy it. The deeper we get into data, the happier we are. An important tool for getting into data (and thus being a happier data analyst) is SQL.

Ignacio L. Bisso

How to Number Rows in an SQL Result Set

Have you ever needed to add a sequential number to the records in the result of an SQL query? It’s not as simple as it looks! Find out how to do it correctly in this article. To number rows in a result set, you have to use an SQL window function called ROW_NUMBER(). This function assigns a sequential integer number to each result row. However, it can also be used to number records in different ways, such as by subsets.

Marija Ilic

Why Should I Learn SQL Window Functions?

SQL has been around for more than 25 years. Its basic syntax – like SELECT, WHERE, GROUP BY, HAVING, and ORDER BY – is well known. But is there something besides traditional SQL? Can you go beyond the basics and improve your skills? The answer to this question is yes. There is a modern version of SQL. In this article, we are going to dive into the modern version and learn about SQL window functions.

LearnSQL.com Team

SQL Course of the Month – Window Functions

Spring has come. It is nicer outside. It's a great time to learn something new. Which course should you choose? In April, it's worth it to bet on SQL window functions. What are they, what are they used for, and why are they worth knowing? I asked Agnieszka Kozubek-Krycuń, Chief Content Officer at Vertabelo, these questions. She has a PhD in mathematics and over 10 years’ experience teaching mathematics and computer science at the University of Warsaw.

LearnSQL.com Team

SQL Window Functions Cheat Sheet

Unlock the full potential of SQL with our comprehensive Window Functions Cheat Sheet! This indispensable guide is designed to elevate your analytics capabilities and make complex data manipulations effortlessly accessible. Welcome to the ultimate resource for mastering SQL window functions - the SQL Window Functions Cheat Sheet! This invaluable resource provides you with the essential syntax, a comprehensive list of window functions, and real-life examples to enhance your SQL skills and analytics capabilities.

Dorota Wdzięczna

What Is a SQL Running Total and How Do You Compute It?

The SQL running total is a very common pattern, used frequently in finance and in trend analysis. In this article, you’ll learn what a running total is and how to write a SQL query to compute it. The best way to learn SQL is through practice. LearnSQL.com offers over 30 interactive SQL courses at various level of difficulty. Each course is interactive: there is a little bit of reading, followed by an exercise to practice what you've just read.

Tihomir Babic

When Do I Use SQL Window Functions?

Have you heard one of your colleagues bragging about using SQL window functions? Do you know basic SQL but not much, or anything, about window functions? If you want to learn about window functions and how to use them in a business context, you’re in the right place! This happens often. You reach a certain level of knowledge, and you feel like the king (or queen) of the world.

Marija Ilic

Analyze Time Series COVID-19 Data with Window Functions

The COVID-19 pandemic really changed the way we live. No matter where you are, you're being impacted by this unpredictable virus. We do not know what the future holds, but we do believe in science and in better days to come. One thing that makes me as a data analyst particularly happy is that we have publicly available data. We can analyze that data and learn something from it. If you want to explore COVID-19 with the help of modern SQL constructions, keep reading — this article is intended for you.

Ignacio L. Bisso

SQL Window Functions vs. GROUP BY: What’s the Difference?

A very common misconception among SQL users is that there is not much difference between SQL window functions and aggregate functions or the GROUP BY clause. However, the differences are very significant. Perhaps the only similar point between GROUP BY and window functions is that both allow you to execute a function (such as AVG, MAX, MIN, or COUNT) on a group of records. I’d say that window functions’ special power is that they allow us to obtain results that otherwise would be almost impossible to achieve.

Agnieszka Kozubek-Krycuń

Why Window Functions Are Not Allowed in WHERE Clauses

Window functions are extremely powerful, but there are a lot of nuances and little details you need to learn about using window functions in order to use them correctly and efficiently. A fairly common question SQL users have is why window functions are not allowed in WHERE. TL;DR: You can use window functions in SELECT and ORDER BY. You cannot use window functions in WHERE, GROUP BY, or HAVING.

Ignacio L. Bisso

SQL Window Functions By Explanation

In our previous post, we explained how SQL window functions work by example. We started with some very simple, basic functions. Let's extend it by explaining subclauses in window functions. SQL window functions are a great way to compute results from a set of rows rather than a single row. As you know from our first article, the "window" in window function refers to the set of rows. We showed you some examples of simple window functions like RANK and LEAD.

Aldo Zelen

Common SQL Window Functions: Positional Functions

Positional SQL window functions deal with data's location in the set. In this post, we explain LEAD, LAG, and other positional functions. SQL window functions allow us to aggregate data while still using individual row values. We've already dealt with ranking functions and the use of partitions. In this post, we'll examine positional window functions, which are extremely helpful in reporting and summarizing data. Specifically, we'll look at LAG, LEAD, FIRST_VALUE and LAST_VALUE.

Aldo Zelen

SQL Window Functions: Partitions and Ranking Functions

You’ve started your mastery of SQL window functions by learning RANK, NTILE, and other basic functions. In this article, we will explain how to use SQL partitions with ranking functions. Mastering SQL window functions (AKA analytical functions) is a bumpy road, but it helps to break the journey into logical stages that build on each other. In the previous Common SQL Functions article, you learned about the various rank functions, which are the most basic form of window functions.

Ignacio L. Bisso

SQL Window Function Example With Explanations

Interested in how window functions work? Scroll down to see our SQL window function example with definitive explanations! SQL window functions are a bit different; they compute their result based on a set of rowsrather than on a single row. In fact, the “window” in “window function” refers to that set of rows. Window functions are similar to aggregate functions, but there is one important difference. When we use aggregate functions with the GROUP BY clause, we “lose” the individual rows.

Aldo Zelen

How to Use Rank Functions in SQL

Want to learn how to use ranking functions in SQL? They are a good choice if you want to start using SQL window functions! Learning about SQL window functions usually comes after you’ve built a foundation in the language, but these powerful functions take your skills up a level. As you master them, you’ll find better ways to solve query problems. When used for business intelligence applications, SQL queries combine data retrieval and advanced computations.

Jeffrey J. Keller

Window Functions: Part Two: Aggregating Data

In part one of this series on SQL Server window functions, the functionality of the OVER() clause was discussed. This article will focus on aggregate functions. The majority of these can be used in conjunction with OVER. Some, like SUM() and AVG(), are widely used. Others — including VARP() and CHECKSUM_AGG() — are not as well-known, but are potentially quite useful. OVER defines the user-specified range on which a function is applied.

Jeffrey J. Keller

Window Functions in SQL Server: Part One: The OVER() Clause

Window functions were first introduced in standard SQL 2003 and then extended in SQL 2008. Since SQL Server 2005, Microsoft has been steadily improving window functions in the database engine. These functions perform an aggregate operation against a user-defined range of rows (the window) and return a detail-level value for each row. Well-known aggregate functions include SUM, AVG, MIN, MAX, and many others. Recent versions of SQL Server have introduced ranking and analytic functions such as LAG, LEAD, RANK, CUME_DIST, and many others.