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JOIN with a multiple-column foreign key. This query lists movie copies from the MOVIE_COPY table that have not been sold (DATE_SOLD IS NULL) and which have been rented (join to the MOVIE_RENTAL table) and not yet returned (RETURNED_DATE IS NULL). Since this table has not been used in a previous example, you may want to refer to the overview of the Video Sample database that appears at the end of 1 to see how it ts into the database schema. Also, keep in mind that, as of this writing, only Oracle and MySQL support this syntax.
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SELECT MOVIE_ID, COPY_NUMBER, DUE_DATE FROM MOVIE_COPY JOIN MOVIE_RENTAL USING (MOVIE_ID, COPY_NUMBER) WHERE DATE_SOLD IS NULL AND RETURNED_DATE IS NULL; MOVIE_ID COPY_NUMBER DUE_DATE --------- ----------- ---------2 2 02/27/2005 3 2 03/04/2005 5 1 02/27/2005 5 2 02/19/2005 10 1 03/04/2005 17 1 03/04/2005 6 rows selected.
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Just to illustrate how much typing the JOIN clause saves, here is the prior query rewritten using join conditions in the WHERE clause. Note that you now must qualify all references to the MOVIE_ID and COPY_NUMBER columns to avoid ambiguous column references. This statement will give you exactly the same result as the previous example:
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SELECT A.MOVIE_ID, A.COPY_NUMBER, DUE_DATE FROM MOVIE_COPY A, MOVIE_RENTAL B WHERE A.MOVIE_ID = B.MOVIE_ID AND A.COPY_NUMBER = B.COPY_NUMBER AND DATE_SOLD IS NULL AND RETURNED_DATE IS NULL;
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A natural join is based on all columns in the two tables that have matching column names. In essence, the equijoins you have already seen are also natural joins. However, the syntax for the natural join is much simpler because there is no need to
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name a condition or a column list the columns to be used are understood. Note that, as of this writing, only Oracle and MySQL support this form of join syntax. Some descriptions of the natural join that you might read elsewhere indicate that the column list being selected has something to do with the de nition of a natural join. Actually, it doesn t you can select whatever columns you wish, but like all SELECT statements, no two columns in the result set may have the same column name, so you have to rename any duplicate column names using a column alias. Here is the join of MOVIE and MOVIE_GENRE, rewritten as a natural join:
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SELECT MOVIE_ID, MOVIE_GENRE_DESCRIPTION AS GENRE, MOVIE_TITLE FROM MOVIE NATURAL JOIN MOVIE_GENRE ORDER BY MOVIE_ID;
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By the way, joins can involve more than two tables. The following example shows a natural join that gets the MOVIE_ID from the MOVIE table, the MOVIE_GENRE_DESCRIPTION from the MOVIE_GENRE table, and the MPAA_RATING_DESCRIPTION from the MPAA_RATING table for the rst ve movies in the MOVIE table. Note the use of the WHERE clause to eliminate unwanted rows from the MOVIE table. The text for the RATING_DESC column was allowed to wrap to new lines in the query results because of its length. The example actually speci es two joins. The rst JOIN clause directs the SQL engine to rst join the MOVIE and MOVIE_GENRE tables, and the second JOIN clause directs it to take those joined rows (essentially an intermediate result set) and join them to the MPAA_RATING table.
SELECT MOVIE_ID, MOVIE_GENRE_DESCRIPTION AS GENRE, MPAA_RATING_CODE AS RATING, MPAA_RATING_DESCRIPTION AS RATING_DESC FROM MOVIE NATURAL JOIN MOVIE_GENRE NATURAL JOIN MPAA_RATING WHERE MOVIE_ID < 6 ORDER BY MOVIE_ID; MOVIE_ID GENRE RATING RATING_DESC --------- --------------------- ------ -------------------1 Drama R Under 17 requires ac companying parent or adult guardian 2 Action and Adventure R Under 17 requires ac companying parent or adult guardian
3 Comedy PG-13
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