Encyclopedia of Database Systems

2018 Edition
| Editors: Ling Liu, M. Tamer Özsu

Window Operator in RDBMS

  • Chee-Yong ChanEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_80628


Analytic OLAP functions; Window functions; Window operator


Window functions, also known as analytic OLAP functions, were introduced in SQL to simplify the formulation of many useful queries that require computations such as ranking, cumulative sums, and moving averages. Window functions are most commonly used in the select clause of SQL queries to compute additional column values for each output tuple, and they could also be used in the order-by clause to compute column values for ordering.

As an example, consider a relation Sales (branch, day, amount) which records the daily sales figures for each branch of a company. To compute the seven-day moving average sales for each branch, one could express the query using a self-join as follows:


s1.branch, s1.day, s1.amount,


(SELECT AVG(x.amount) FROM


(SELECT s2.amount AS amount


FROM Sales s2


WHERE s2.branch = s1.branch


AND s2.day <= s1.day




7) AS x) AS movingAvg


Sales s1

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Authors and Affiliations

  1. 1.National University of SingaporeSingaporeSingapore