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How IT Works A Primer On SQL Joins
Joshua Trupin


One of the trickiest things about learning SQL is mastering how various JOIN statements differ in the ways they combine data from multiple data tables. There are three types of joins: inner, outer, and cross. In addition, there are three types of outer joins: left, right, and full. It can be frustrating trying to keep them differentiated, so here's a quick guide. All of the following examples involve joining the authors and publishers tables in the Pubs sample database included with SQL Server.

Inner Joins
In an inner join, records from two tables are combined and added to a query's results only if the values of the joined fields meet certain specified criteria. If you use an inner join to combine the authors and publishers tables based on their city and state columns, the result would be a list of all authors who live in the same city as a publisher (see Figure 1):
USE pubs
SELECT a.au_fname, a.au_lname, p.pub_name 
FROM authors AS a INNER JOIN publishers AS p
ON a.city = p.city
AND a.state = p.state
ORDER BY a.au_lname ASC, a.au_fname ASC

au_fname au_lname pub_name
Abraham Bennet Algodata Infosystems
Cheryl Carson Algodata Infosystems

Outer Joins
An outer join returns all rows from the joined tables whether or not there's a matching row between them. The ON clause supplements the data rather than filtering it. The three types of outer joins, left, right, and full, indicate the main data's source.

Left Outer Joins
When you use a left outer join to combine two tables, all the rows from the left-hand table are included in the results. So, for the authors and publishers tables, the result will include a list of all authors along with a publisher's name column. If a publisher exists in the author's city, it's listed. Otherwise, the field in the publisher's column is set to NULL (see Figure 2):
USE pubs
SELECT a.au_fname, a.au_lname, p.pub_name
FROM authors a LEFT OUTER JOIN publishers p
ON a.city = p.city
ORDER BY p.pub_name ASC, a.au_lname ASC, a.au_fname ASC

au_fname au_lname pub_name
Reginald Blotchet-Halls NULL
Michel DeFrance NULL
Innes del Castillo NULL
Ann Dull NULL
Marjorie Green NULL
Morningstar Greene NULL
Burt Gringlesby NULL
Sheryl Hunter NULL
Livia Karsen NULL
Charlene Locksley NULL
Stearns MacFeather NULL
Heather McBadden NULL
Michael O'Leary NULL
Sylvia Panteley NULL
Albert Ringer NULL
Anne Ringer NULL
Meander Smith NULL
Dean Straight NULL
Dirk Stringer NULL
Johnson White NULL
Akiko Yokomoto NULL
Abraham Bennet Algodata Infosystems
Cheryl Carson Algodata Infosystems

Right Outer Joins
A right outer join is conceptually the same as a left outer join except that all the rows from the right-hand table are included in the results. They can be included more than once, if more than one author exists in the publisher's city. If no author lives in the publisher's city, the author name fields are set to NULL (see Figure 3):
USE pubs
SELECT a.au_fname, a.au_lname, p.pub_name
FROM authors AS a RIGHT OUTER JOIN publishers AS p
ON a.city = p.city
ORDER BY p.pub_name ASC, a.au_lname ASC, a.au_fname ASC

au_fname au_lname pub_name
Abraham Bennet Algodata Infosystems
Cheryl Carson Algodata Infosystems
NULL NULL Binnet & Hardley
NULL NULL Five Lakes Publishing
NULL NULL GGG&G
NULL NULL Lucerne Publishing
NULL NULL New Moon Books
NULL NULL Ramona Publishers
NULL NULL Scootney Books

Full Outer Join
As you might have gathered, a full outer join retrieves all the rows from both joined tables. It returns all of the paired rows where the join condition is true, plus the unpaired rows from each table concatenated with NULL rows from the other table. You usually won't want to use one of these.

Cross Join
A cross join returns not the sum but the product of two tables. Each row in the left-hand table is matched up with each row in the right-hand table. It's the set of all possible row combinations, without any filtering, as shown here:
USE pubs
SELECT au_fname, au_lname, pub_name
FROM authors CROSS JOIN publishers 
ORDER BY au_lname DESC
The resultset contains 184 rows (authors has 23 rows, and publishers has 8; therefore, 23 × 8 = 184). The first 11 rows look like Figure 4.

au_fname au_lname pub_name
Akiko Yokomoto New Moon Books
Akiko Yokomoto Binnet & Hardley
Akiko Yokomoto Algodata Infosystems
Akiko Yokomoto Five Lakes Publishing
Akiko Yokomoto Ramona Publishers
Akiko Yokomoto GGG&G
Akiko Yokomoto Scootney Books
Akiko Yokomoto Lucerne Publishing
Johnson White New Moon Books
Johnson White Binnet & Hardley
Johnson White Algodata Infosystems
However, if you add a WHERE clause (like WHERE authors.city = publishers.city), a cross join functions as an inner join—it uses the condition to filter all possible row combinations down to the ones you want (see Figure 5):
USE pubs
SELECT au_fname, au_lname, pub_name
FROM authors CROSS JOIN publishers 
WHERE authors.city = publishers.city
ORDER BY au_lname DESC

au_fname au_lname pub_name
Cheryl Carson Algodata Infosystems
Abraham Bennet Algodata Infosystems

Joshua Trupin is the Executive Editor of TechNet Magazine and MSDN Magazine.
© 2008 Microsoft Corporation and CMP Media, LLC. All rights reserved; reproduction in part or in whole without permission is prohibited.
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