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 rows rather 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. We can’t mix attributes from an individual row with the results of an aggregate function; the function is performed on the rows as an entire group. This is not the case when we use SQL window functions: we can generate a result set with some attributes of an individual row together with the results of the window function. This is good for new SQL developers to keep in mind. So let’s examine a simple SQL window function example in action.

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SQL Window Function Example

Window functions can be called in the SELECT statement or in the ORDER BY clause. However, they can never be called in the WHERE clause. You’ll notice that all the examples in this article call the window function in the SELECT column list.

Let’s go to the first SQL window function example. We will use the “Employee” table:

We will begin with RANK, which is one of the simplest SQL window functions example. It returns the position of any row inside the partition. Let’s use it to rank salaries within departments:

SELECT	
		RANK() OVER (PARTITION BY department ORDER BY salary DESC) 
			AS dept_ranking,
		department,
		employee_id, 
		full_name, 
		salary
FROM employee;

We can see the results below:

What if we want to have the same report but with all the top-ranking employees first, then all second-ranking employees, and so on? Well, we’ll give you this challenge to figure out on your own.  Share your ideas in the comments section!

Proceeding with our SQL window function example, let’s find out where each employee’s salary ranks in relation to the top salary of their department. This calls for a math expression, like:

employee_salary / max_salary_in_depth

The next query will show all employees ordered by the above metric; the employees with the lowest salary (relative to their highest departmental salary) will be listed first:

SELECT
  employee_id, 
  full_name, 
  department,
  salary,
  salary / MAX(salary) OVER (PARTITION BY department ORDER BY salary DESC) 
    AS salary_metric
FROM employee
ORDER BY 5;

Interested in learning SQL window functions? Try out our interactive Window Functions course!

Another SQL Window Function Example

Let’s switch from an employee-salary database to the following train schedule database:

Suppose we want to add a new column called “time to next station”. To obtain this value, we subtract the station times for pairs of contiguous stations. We can calculate this value without using a SQL window function, but that can be very complicated. It’s simpler to do it using the LEAD window function. This function compares values from one row with the next row to come up with a result.  In this case, it compares the values in the “time” column for a station with the station immediately after it.

So, here we have another SQL window function example, this time for the train schedule:

SELECT 
	train_id, 
	station,
	time as "station_time",
	lead(time) OVER (PARTITION BY train_id ORDER BY time) - time 
		AS time_to_next_station
FROM train_schedule;

Note that we calculate the LEAD window function by using an expression involving an individual column and a window function; this is not possible with aggregate functions.

Here are the results of that query:

In the next example, we will add a new column that shows how much time has elapsed from the train’s first stop to the current station. We will call it “elapsed travel time”. The MIN window function will obtain the trip’s start time and we will subtract the current station time. Here’s the next SQL window function example

SELECT 	
	train_id, 
	station,
	time as "station_time",
	time - min(time) OVER (PARTITION BY train_id ORDER BY time) 	
								AS elapsed_travel_time,
	lead(time) OVER (PARTITION BY train_id ORDER BY time) - time 
								AS time_to_next_station
FROM train_schedule;

Notice the new column in the result table:

Discover More SQL Window Function Examples!

Window functions are not well-known aspects of SQL, but their power and flexibility make them very important. There are clauses (e.g. PARTITION BY and window frame) and topics that we didn’t cover in this article, but don’t let that stop you! You can learn more and find other SQL window functions examples using our blog and the Window Functions course in LearnSQL.com. Start today!