Joining Three or More Tables

Joining Three or More Tables

Although each join specification joins only two tables, FROM clauses can contain multiple join specifications. This allows many tables to be joined for a single query.

The titleauthor table of the pubs database offers a good example of a situation in which joining more than two tables is helpful. This Transact-SQL query finds the titles of all books of a particular type and the names of their authors:

USE pubs
SELECT a.au_lname, a.au_fname, t.title
FROM authors a INNER JOIN titleauthor ta
ON a.au_id = ta.au_id JOIN titles t
ON ta.title_id = t.title_id
WHERE t.type = 'trad_cook'
ORDER BY t.title ASC

Here is the result set:

au_lname          au_fname             title
----------------- -------------------- ---------- 

Blotchet-Halls      Reginald          Fifty Years in Buckingham Palace
Kitchens
Panteley            Sylvia            Onions, Leeks, and Garlic: 
Cooking Secrets of the Mediterranean
O'Leary             Michael            Sushi, Anyone?
Gringlesby          Burt               Sushi, Anyone?
Yokomoto            Akiko              Sushi, Anyone?
(5 row(s) affected)

Notice that one of the tables in the FROM clause, titleauthor, does not contribute any columns to the results. Also, none of the joined columns, au_id and title_id, appear in the results. Nonetheless, this join is possible only by using titleauthor as an intermediate table.

The middle table of the join (the titleauthor table) can be called the translation table or intermediate table, because titleauthor is an intermediate point of connection between the other tables involved in the join.

When there is more than one join operator in the same statement, either to join more than two tables or to join more than two pairs of columns, the join expressions can be connected with AND or with OR.

See Also http://msdn2.microsoft.com/en-us/library/aa213227(SQL.80).aspx