using grouping sets in sql server 2008 tech republic

Using Grouping Sets in SQL Server 2008 | TechRepublic

http://www.techrepublic.com/blog/datacenter/using-grouping-sets-in-sql-server-2008/456[08/29/2012 3:43:37 PM]

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Using Grouping Sets in SQLServer 2008By Tim ChapmanOctober 3, 2008, 8:23 AM PDT

Takeaway: A neat new feature in SQL Server 2008 is the GROUPING SETS clause, whichallows you to easily specify combinations of field groupings in your queries to see different levels ofaggregated data. Today well look at how you can use the new SQL Server 2008 GROUPINGSETS clause to aggregate your data.

GROUP BYThe GROUP BY clause is a SQL language construct used to priovide summary data for columnreturned in a SELECT statement. This functionality groups values from specified fields together,providing a single record of distinct values for each group. To illustrate the how GROUP BYworks, lets look at an example. Use the script below to create and load the SalesHistory table.

IF OBJECT_ID('SalesHistory')>0

DROP TABLE SalesHistory;

GO

CREATE TABLE [dbo].[SalesHistory]

( [SaleID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY,

[Product] [char] (150) NULL,

[SaleDate] [datetime] NULL,

[SalePrice] [money] NULL

)

GO

DECLARE @i SMALLINT

SET @i = 1

WHILE (@i



VALUES ('BigScreen', DATEADD(mm, @i, '3/11/1927'), DATEPART(ms, GETDATE()) + (@i + 13))

INSERT INTO SalesHistory

(Product, SaleDate, SalePrice)

VALUES ('PoolTable', DATEADD(mm, @i, '3/11/1908'), DATEPART(ms, GETDATE()) + (@i + 29))

SET @i = @i + 1

END

I can use the GROUP by clause to retrieve the average SalePrice per Product.

SELECT Product, AVG(SalePrice) AS AverageSalePrice

FROM SalesHistory

GROUP BY Product

Notice that the GROUP BY clause returns a distinct Product list, followed by the average of eachSalePrice value.

The only drawback of the GROUP BY clause is that it only really allows you to specifyaggregations for a single set of database columns. The new SQL Server 2008 GROUPING SETSclause expands upon the GROUP BY functionality allowing you to specify different fieldcombinations to return aggregate data.

GROUPING SETS

This functionality can enhance reporting requirements and data analysis by allowing you to retrieveaggregation data through one statement, rather than several distinct queries. GROUPING SETSalso allows for Grand total data for the entire set of data, or just for sections of aggregations.

The following query uses the GROUPING SETS clause on the SalesHistory table. This queryreturns 2 different sets of aggregations; one at the Product level, and a Grand total level, denotedby the () in the GROUPING SETS clause.

SELECT Product, SUM(SalePrice) AS TotalSales, COUNT(*) AS SaleCount

FROM SalesHistory

GROUP BY GROUPING SETS((Product),())

ORDER BY Product DESC, TotalSales DESC, SaleCount DESC

Notice the last row returned in the above screenshot. This is the Grant total record I mentionedabove.

In the following query, I use three separate field sets in my GROUPING SETS clause. Doing socreate three totally different sets of aggregations; one by Product, one by the SalesTier (calculatedvia a subquery), and one Grand total column. Note that the order in which the grouping setsappear in the GROUPING SETS clause is not important.

SELECT

Product, SalesTier, TotalSales = SUM(SalePrice), AverageSalePrice = AVG(SalePrice)

FROM (

SELECT

Product,

SalesTier =

CASE WHEN SalePrice BETWEEN 0 AND 500 THEN 'Tier 1'

WHEN SalePrice BETWEEN 501 AND 750 THEN 'Tier 2'


WHEN SalePrice > 1000 THEN 'Tier 4'

END,

SalePrice

FROM SalesHistory

) a

GROUP BY



GROUPING SETS((Product), (SalesTier), ())

ORDER BY Product DESC, SalesTier DESC

You can achieve the same results as the above query using typical TSQL constructs, but it ismuch more cumbersome to do so, as I show in the following script.

IF OBJECT_ID('tempdb..#SalesResults') IS NOT NULL

DROP TABLE #SalesResults

CREATE TABLE #SalesResults

(

Product VARCHAR(10),

SalesTierVARCHAR(10),Next Time

TotalSales MONEY,

AverageSalePrice MONEY

)

INSERT INTO #SalesResults(Product, SalesTier, TotalSales, AverageSalePrice)

SELECT

Product, NULL, SUM(SalePrice), AVG(SalePrice)

FROM

SalesHistory

GROUP BY Product

UNION ALL

SELECT

NULL,

SalesTier,

SUM(SalePrice),

AVG(SalePrice)

FROM

(

SELECT

SalesTier =

CASE




WHEN SalePrice > 1000 THEN 'Tier 4'

END,

SalePrice

FROM SalesHistory

) a

GROUP BY SalesTier

UNION ALL

SELECT

NULL, NULL, SUM(SalePrice), AVG(SalePrice)

FROM SalesHistory

SELECT *

FROM #SalesResults

Next Time



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Today I took a look at the usefulness of the new GROUPING SETS clause in SQL Server 2008.The results of the GROUPING SETS clause can be achived in other ways, but it takes moreprogramming work to do so. Next time I will take a look at SQL Server 2008 enhancements to theCUBE and ROLLUP; two constructs that can produce similar results to the GROUPING SETSclause, but in a different manner.

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RE: Using Grouping Sets in SQL Server 2008chapman.tim@... 7th Oct 2008

Looks to be...thanks for noticing. I'll get with TR to get them posted.

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Missing screen shots?MikeSQLDBA 6th Oct 2008

Interesting article, but are the screen shots missing?

0Votes

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