Window Functions

Overview

A window function operates on a group ("window") of related rows.

For each input row, a window function returns one output row that depends on the specific row passed to the function and the values of the other rows in the window.

There are two main types of order-sensitive window functions:

• Rank-related functions: Rank-related functions list information based on the "rank" of a row. For example, ranking stores in descending order by profit per year, the store with the most profit will be ranked 1, and the second-most profitable store will be ranked 2, and so on.

• Window frame functions: Window frame functions enable you to perform rolling operations, such as calculating a running total or a moving average, on a subset of the rows in the window.

List of Functions that Support Windows

The list below shows all the window functions.

Function Name	Category	Window	Window Frame	Notes
ARRAY_AGG	General	✔
AVG	General	✔	✔
AVG_IF	General	✔	✔
COUNT	General	✔	✔
COUNT_IF	General	✔	✔
COVAR_POP	General	✔
COVAR_SAMP	General	✔
MAX	General	✔	✔
MAX_IF	General	✔	✔
MIN	General	✔	✔
MIN_IF	General	✔	✔
STDDEV_POP	General	✔	✔
STDDEV_SAMP	General	✔	✔
MEDIAN	General	✔	✔
QUANTILE_CONT	General	✔	✔
QUANTILE_DISC	General	✔	✔
KURTOSIS	General	✔	✔
SKEWNESS	General	✔	✔
SUM	General	✔	✔
SUM_IF	General	✔	✔
CUME_DIST	Rank-related	✔
PERCENT_RANK	Rank-related	✔	✔
DENSE_RANK	Rank-related	✔	✔
RANK	Rank-related	✔	✔
ROW_NUMBER	Rank-related	✔
NTILE	Rank-related	✔
FIRST_VALUE	Rank-related	✔	✔
FIRST	Rank-related	✔	✔
LAST_VALUE	Rank-related	✔	✔
LAST	Rank-related	✔	✔
NTH_VALUE	Rank-related	✔	✔
LEAD	Rank-related	✔
LAG	Rank-related	✔

Window Syntax

<function> ( [ <arguments> ] ) OVER ( { named window | inline window } )

named window ::=
    { window_name | ( window_name ) }

inline window ::=
    [ PARTITION BY <expression_list> ]
    [ ORDER BY <expression_list> ]
    [ window frame ]

The named window is a window that is defined in the WINDOW clause of the SELECT statement, eg: SELECT a, SUM(a) OVER w FROM t WINDOW w AS ( inline window ).

The <function> is one of (aggregate function, rank function, value function).

The OVER clause specifies that the function is being used as a window function.

The PARTITION BY sub-clause allows rows to be grouped into sub-groups, for example by city, by year, etc. The PARTITION BY clause is optional. You can analyze an entire group of rows without breaking it into sub-groups.

The ORDER BY clause orders rows within the window.

The window frame clause specifies the window frame type and the window frame extent. The window frame clause is optional. If you omit the window frame clause, the default window frame type is RANGE and the default window frame extent is UNBOUNDED PRECEDING AND CURRENT ROW.

Window Frame Syntax

window frame can be one of the following types:

cumulativeFrame ::=
    {
       { ROWS | RANGE } BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
     | { ROWS | RANGE } BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
    }

slidingFrame ::=
    {
       ROWS BETWEEN <N> { PRECEDING | FOLLOWING } AND <N> { PRECEDING | FOLLOWING }
     | ROWS BETWEEN UNBOUNDED PRECEDING AND <N> { PRECEDING | FOLLOWING }
     | ROWS BETWEEN <N> { PRECEDING | FOLLOWING } AND UNBOUNDED FOLLOWING
    }

Examples

Create the table

CREATE TABLE employees (
  employee_id INT,
  first_name VARCHAR,
  last_name VARCHAR,
  department VARCHAR,
  salary INT
);

Insert data

INSERT INTO employees (employee_id, first_name, last_name, department, salary) VALUES
  (1, 'John', 'Doe', 'IT', 75000),
  (2, 'Jane', 'Smith', 'HR', 85000),
  (3, 'Mike', 'Johnson', 'IT', 90000),
  (4, 'Sara', 'Williams', 'Sales', 60000),
  (5, 'Tom', 'Brown', 'HR', 82000),
  (6, 'Ava', 'Davis', 'Sales', 62000),
  (7, 'Olivia', 'Taylor', 'IT', 72000),
  (8, 'Emily', 'Anderson', 'HR', 77000),
  (9, 'Sophia', 'Lee', 'Sales', 58000),
  (10, 'Ella', 'Thomas', 'IT', 67000);

Example 1: Ranking employees by salary

In this example, we use the RANK() function to rank employees based on their salaries in descending order. The highest salary will get a rank of 1, and the lowest salary will get the highest rank number.

SELECT employee_id, first_name, last_name, department, salary, RANK() OVER (ORDER BY salary DESC) AS rank
FROM employees;

Result:

employee_id	first_name	last_name	department	salary	rank
3	Mike	Johnson	IT	90000	1
2	Jane	Smith	HR	85000	2
5	Tom	Brown	HR	82000	3
8	Emily	Anderson	HR	77000	4
1	John	Doe	IT	75000	5
7	Olivia	Taylor	IT	72000	6
10	Ella	Thomas	IT	67000	7
6	Ava	Davis	Sales	62000	8
4	Sara	Williams	Sales	60000	9
9	Sophia	Lee	Sales	58000	10

Example 2: Calculating the total salary per department

In this example, we use the SUM() function with PARTITION BY to calculate the total salary paid per department. Each row will show the department and the total salary for that department.

SELECT department, SUM(salary) OVER (PARTITION BY department) AS total_salary
FROM employees;

Result:

department	total_salary
HR	244000
HR	244000
HR	244000
IT	304000
IT	304000
IT	304000
IT	304000
Sales	180000
Sales	180000
Sales	180000

Example 3: Calculating a running total of salaries per department

In this example, we use the SUM() function with a cumulative window frame to calculate a running total of salaries within each department. The running total is calculated based on the employee's salary ordered by their employee_id.

SELECT employee_id, first_name, last_name, department, salary, 
       SUM(salary) OVER (PARTITION BY department ORDER BY employee_id
                         ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS running_total
FROM employees;

Result:

employee_id	first_name	last_name	department	salary	running_total
2	Jane	Smith	HR	85000	85000
5	Tom	Brown	HR	82000	167000
8	Emily	Anderson	HR	77000	244000
1	John	Doe	IT	75000	75000
3	Mike	Johnson	IT	90000	165000
7	Olivia	Taylor	IT	72000	237000
10	Ella	Thomas	IT	67000	304000
4	Sara	Williams	Sales	60000	60000
6	Ava	Davis	Sales	62000	122000
9	Sophia	Lee	Sales	58000	180000