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  SQL Server Tips by Gama and Naughter

Self join

This is a particular case when one table joins to itself, with one or two aliases to avoid confusion. A self join can be of any type, as long as the joined tables are the same.

Example: get the names of the authors who live in the same city, only if two or more live in that city, plus the names of the cities.

SELECT DISTINCT TOP 100 PERCENT dbo.authors.au_lname, dbo.authors.au_fname, dbo.authors.city
FROM dbo.authors INNER JOIN
dbo.authors authors_1 ON dbo.authors.city = authors_1.city AND
dbo.authors.au_fname + ' ' + dbo.authors.au_lname <> authors_1.au_fname +' ' + authors_1.au_lname
ORDER BY dbo.authors.city, dbo.authors.au_lname


The INNER JOIN will filter by the authors who live in the same city and have different names. Concatenating the first and last names simplifies the logic for comparing the names for readability but this technique does not necessarily improve performance.


The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter

 http://www.rampant-books.com/book_2005_2_sql_server_external_procedures.htm  

 

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