Introduction to Window Functions Kathi Kellenberger, MVP Linchpin People auntkathisql.com

History 2005: Window functions introduced 2012: Window functions enhanced

Agenda What are window functions? Ranking functions Window aggregates 2012 Enhancements: ORDER BY 2012 Enhancements: Framing Analytic Functions Offset Functions Statistical Functions Making Business Intelligent

What are window functions? Function performs over a SET (window of the results) How to identify a window function The OVER clause defines the window Where to put window functions SELECT and ORDER BY clauses only Important! The FROM, WHERE, GROUP BY and HAVING clauses operate BEFORE the window functions Partitions NOT the same as GROUP BY

Ranking functions Available since 2005 ROW_NUMBER() A unique # over the window RANK() Deals with ties DENSE_RANK() NTILE() Divide rows into buckets

ROW_NUMBER() example CustomerID OrderID Total Row_Number() Over(Order by OrderID) 1 101 100 2 102 40 103 11 3 104 432 4 105 2000 5 106 300 6 107 674 7 108 76 8 109 234 9 110 889 10

ROW_NUMBER() example CustomerID OrderID TotalDue Row_Number() Over(Partition by CustomerID Order by OrderID) 1 100 5 2000 2 6 300 3 40 11 4 432 7 674 9 234 10 889 8 76

Ranking Functions Demo

Window aggregate functions Your favorite aggregates with no GROUP BY Add aggregate function to a non-aggregate query Calculate over the window: the entire result set or partition Pre-2012 functionality NOT optimized for performance

Window aggregate example CustomerID OrderID TotalDue Sum(TotalDue) Over() 1 100 4990 5 2000 6 300 2 40 3 11 4 432 7 674 9 234 10 889 8 76

Window aggregate example CustomerID OrderID TotalDue Sum(TotalDue) Over(Partition by CustomerID) 1 100 2400 5 2000 6 300 2 40 51 3 11 4 432 7 674 1797 9 234 10 889 8 76 310

Window Aggregates Demo

2012 enhancements ORDER BY clause added to window aggregates Performance optimization Even further define the window with ROWS and RANGE: FRAMING

Window aggregate ORDER BY example CustomerID OrderID TotalDue Sum(TotalDue) Over(Order by OrderID) 1 100 2 40 140 3 11 151 4 432 583 5 2000 2583 6 300 2883 7 674 3557 8 76 3633 9 234 3867 10 889 4756 4990

Running Total Demo

FRAME: ROWS and RANGE Further define window Terms UNBOUNDED PRECEDING UNBOUNDED FOLLOWING CURRENT ROW # PRECEDING # FOLLOWING RANGE is not fully implemented and can hurt performance

Framing Rows between unbounded preceding and current row

Framing Rows between current row and unbounded following

Framing Rows between 2 preceding and current row

Framing Rows between 5 preceding and 2 following

Framing Demo

The Analytic Functions LAG() and LEAD() Look back or look forward FIRST_VALUE() and LAST_VALUE() The very first or very last PERCENT_RANK() and CUME_DIST() Calculate ranking PERCENTILE_DISC() and PERCENTILE_CONT() Given a percent, find the value

Analytic Functions Demo

Resources https://www.pluralsight.com/courses/tsql-window-functions