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Microsoft® SQL Server 2008 R2 Unleashed
book

Microsoft® SQL Server 2008 R2 Unleashed

by Ray Rankins, Paul Bertucci, Chris Gallelli, Alex T. Silverstein
September 2010
Intermediate to advanced content levelIntermediate to advanced
1704 pages
111h 8m
English
Sams
Content preview from Microsoft® SQL Server 2008 R2 Unleashed

Queries with UNION

When you specify UNION in a query, SQL Server merges the result sets, applying one of the merge or concatenation operators with sorting strategies to remove any duplicate rows. Figure 35.25 shows an example similar to the OR strategy where the rows are concatenated and then sorted to remove any duplicates.

Figure 35.25 An execution plan for a UNION query.

image

If you specify UNION ALL in a query, SQL Server simply appends the result sets together. No intermediate sorting or merge step is needed to remove duplicates. Figure 35.26 shows the same query as in Figure 35.25, except that a UNION ALL is specified.

Figure 35.26 An execution ...

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Publisher Resources

ISBN: 9780768696585Purchase book