1. T-SQL Window Functions in Microsoft SQL Server Denali
Itzik Ben-Gan, SolidQ

2. Window Functions Window Functions, Described
Limitations of Alternatives
Windowing Concepts
Window Functions Supported in SQL Server
Optimization of Window Functions
© 2011 Itzik Ben-Gan, SolidQ

3. Window Functions, Described
Functions operating on a window (set) of rows defined by an OVER clause
Types of functions:
Ranking
Aggregate
Distribution
SELECT empid, ordermonth, qty,
SUM(qty) OVER(PARTITION BY empid
ORDER BY ordermonth
ROWS BETWEEN UNBOUNDED PRECEDING
AND CURRENT ROW) AS runqty
FROM Sales.EmpOrders;
© 2011 Itzik Ben-Gan, SolidQ

4. Limitations of Alternatives
Grouped queries:
Force all calculations in the query to be done in context of groups
Subqueries:
Start from scratch
Each requires separate visit to data
Window functions:
Starting point is underlying query’s result set (hence allowed only in SELECT and ORDER BY)
Can utilize same access to data for multiple functions
© 2011 Itzik Ben-Gan, SolidQ

5. Windowing Concepts Partitioning Ordering Framing
© 2011 Itzik Ben-Gan, SolidQ

6. Partitioning
Filters only rows with same values in partitioning elements
SELECT orderid, custid, val,
CAST(100. * val / SUM(val) OVER() AS NUMERIC(5, 2)) AS pctall,
CAST(100. * val / SUM(val) OVER(PARTITION BY custid) AS NUMERIC(5, 2)) AS pctcust
FROM Sales.OrderValues AS O1;
© 2011 Itzik Ben-Gan, SolidQ

7. Ordering and Framing Window order clause gives meaning to framing
Framing further filters rows within partition by identifying bounds
Window frame units:
ROWS: offset in terms of number of rows
RANGE: offset in terms of values
© 2011 Itzik Ben-Gan, SolidQ

8. Framing, ROWS SELECT empid, ordermonth, qty,
SUM(qty) OVER(PARTITION BY empid
ORDER BY ordermonth
ROWS BETWEEN UNBOUNDED PRECEDING
AND CURRENT ROW) AS runqty
FROM Sales.EmpOrders; -- Same as: ROWS UNBOUNDED PRECEDING
© 2011 Itzik Ben-Gan, SolidQ

9. Framing, ROWS SELECT empid, ordermonth,
MAX(qty) OVER(PARTITION BY empid
ORDER BY ordermonth
ROWS BETWEEN 1 PRECEDING
AND 1 PRECEDING) AS prvqty,
qty AS curqty,
ROWS BETWEEN 1 FOLLOWING
AND 1 FOLLOWING) AS nxtqty,
AVG(qty) OVER(PARTITION BY empid
AND 1 FOLLOWING) AS avgqty
FROM Sales.EmpOrders;
© 2011 Itzik Ben-Gan, SolidQ

10. Framing, RANGE
-- this example is not supported in Denali SELECT empid, ordermonth, qty, SUM(qty) OVER(PARTITION BY empid ORDER BY ordermonth RANGE BETWEEN INTERVAL '2' MONTH PRECEDING AND CURRENT ROW) AS sum3month FROM Sales.EmpOrders;
SQL Server Denali supports only UNBOUNDED and CURRENT ROW delimiters
When ORDER BY specified but no explicit frame, default is RANGE UNBOUNDED PRECEDING
© 2011 Itzik Ben-Gan, SolidQ

11. Window Functions Supported in SQL Server
SQL Server 2005 added initial support:
Ranking calculations
Aggregates with only window partitioning
SQL Server Denali has more complete support:
Aggregates with also window ordering and framing
Offset functions: LAG, LEAD, FIRST_VALUE, LAST_VALUE
Distribution functions: PERCENT_RANK, CUME_DIST, PERCENTILE_CONT, PERCENTILE_DISC
© 2011 Itzik Ben-Gan, SolidQ

12. Examples for Aggregates
-- running sum
SELECT actid, tranid, val,
SUM(val) OVER(PARTITION BY actid
ORDER BY tranid
ROWS BETWEEN UNBOUNDED PRECEDING
AND CURRENT ROW) AS balance
FROM dbo.Transactions;
-- moving average
AVG(val) OVER(PARTITION BY actid
ROWS BETWEEN 3 PRECEDING
AND CURRENT ROW) AS movavg
© 2011 Itzik Ben-Gan, SolidQ

13. Examples for Offset
-- Diff between current order value and prev/next order values
SELECT custid, orderid, orderdate, val,
val - LAG(val) OVER(PARTITION BY custid ORDER BY orderdate, orderid) AS diffprv,
val - LEAD(val) OVER(PARTITION BY custid ORDER BY orderdate, orderid) AS diffnxt
FROM Sales.OrderValues;
-- Diff between current order value and first/last order values
val - FIRST_VALUE(val) OVER(PARTITION BY custid ORDER BY orderdate, orderid) AS difffirst,
val - LAST_VALUE(val) OVER(PARTITION BY custid
ORDER BY orderdate, orderid
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING) AS difflast
© 2011 Itzik Ben-Gan, SolidQ

14. Examples for Rank Distribution
rk = RANK, nr = count, np = rows precede or peer
PERCENT_RANK = (rk - 1) / (nr - 1)
CUME_DIST = np / nr
-- Query Computing PERCENT_RANK and CUME_DIST
SELECT testid, studentid, score,
PERCENT_RANK() OVER(PARTITION BY testid ORDER BY score) AS percentrank,
CUME_DIST() OVER(PARTITION BY testid ORDER BY score) AS cumedist
FROM Stats.Scores;
© 2011 Itzik Ben-Gan, SolidQ

15. Examples for Inverse Distribution
PERCENTILE_DISC = percentile assuming discrete distribution model
PERCENTILE_CONT = percentile assuming continuouse distribution model
-- Query Computing PERCENTILE_DISC and PERCENTILE_CONT
AS FLOAT = 0.5;
SELECT testid, score,
WITHIN GROUP(ORDER BY score) OVER(PARTITION BY testid) AS percentiledisc,
WITHIN GROUP(ORDER BY score) OVER(PARTITION BY testid) AS percentilecont
FROM Stats.Scores;
© 2011 Itzik Ben-Gan, SolidQ

16. Optimization of Window Functions
Indexing Guidelines
POC
Ascending partitioning values
Fast Track with UNBOUNDED PRECEDING
Expanding all frame rows
When frame <= 4 rows
When function isn’t cumulative, e.g., MIN, MAX
When extreme diff between cur, top, bottom > 10,000 or using RANGE will use on-disk spool
Compute two cumulative values
Function is cumulative, e.g., SUM, COUNT
Frame > 4 rows
© 2011 Itzik Ben-Gan, SolidQ

17. More info… SolidQ.com, 5-day Advanced T-SQL Class InsideTSQL.com
SQLMag.com
© 2011 Itzik Ben-Gan, SolidQ

18. For attending this session and PASS SQLRally Nordic 2011, Stockholm
THANK YOU!
For attending this session and PASS SQLRally Nordic 2011, Stockholm