The titleauthor table of pubs2 offers a good example of a situation in which joining more than two tables is helpful. To find the titles of all the books of a particular type and the names of their authors, use:
select au_lname, au_fname, title from authors, titles, titleauthor where authors.au_id = titleauthor.au_id and titles.title_id = titleauthor.title_id and titles.type = "trad_cook"
au_lname au_fname title -------------- ----------- ------------------------ Panteley Sylvia Onions, Leeks, and Garlic: Cooking Secrets of the Mediterranean Blotchet-Halls Reginald Fifty Years in Buckingham Palace Kitchens O’Leary Michael Sushi, Anyone? Gringlesby Burt Sushi, Anyone? Yokomoto Akiko Sushi, Anyone? (5 rows affected)
One of the tables in the from clause, titleauthor, does not contribute any columns to the results. Nor do any of the columns that are joined—au_id and title_id—appear in the results. Nonetheless, this join is possible only by using titleauthor as an intermediate table.
You can also join more than two pairs of columns in the same statement. For example, here is a query that shows the title_id, its total sales and the range in which they fall, and the resulting royalty:
select titles.title_id, total_sales, lorange, hirange, royalty from titles, roysched where titles.title_id = roysched.title_id and total_sales >= lorange and total_sales < hirange
title_id total_sales lorange hirange royalty -------- ----------- ------- ------- ------- BU1032 4095 0 5000 10 BU1111 3876 0 4000 10 BU2075 18722 14001 50000 24 BU7832 4095 0 5000 10 MC2222 2032 2001 4000 12 MC3021 2224 12001 50000 24 PC1035 8780 4001 10000 16 PC8888 4095 0 5000 10 PS1372 375 0 10000 10 PS2091 2045 1001 5000 12 PS2106 111 0 2000 10 PS3333 4072 0 5000 10 PS7777 3336 0 5000 10 TC3218 375 0 2000 10 TC4203 15096 8001 16000 14 TC7777 4095 0 5000 10 (16 rows affected)
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” are almost always connected with and, as in the earlier examples. However, it is also legal to connect them with or.
