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Joins Using the WHERE Clause
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Using the WHERE clause to join tables is very much like using it to eliminate unwanted rows in the result set. However, there are some differences. First, you are comparing a column to another column in the WHERE condition instead of comparing a column value to a constant or expression. Second, when the columns in the two tables have the same name (which is a best practice), you must qualify the column names so the SQL engine can tell which of the two columns is being referenced. While your intention may seem obvious to you, the SQL engine is going to insist that you make unambiguous references to every column you mention in an SQL statement. By the way, this includes not only columns in the WHERE clause, but also elsewhere in the statement, including in the SELECT list. The simplest form of quali er is the table name itself, with a period (dot) used to separate it from the column name. Following is an example of a join speci ed in the WHERE clause with table names used as quali ers. Notice how it selects MOVIE_ID and MOVIE_TITLE from MOVIE and the corresponding genre (GENRE_DESCRIPTION) from MOVIE_GENRE.
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SELECT MOVIE_ID, MOVIE_GENRE_DESCRIPTION AS GENRE, MOVIE_TITLE FROM MOVIE, MOVIE_GENRE WHERE MOVIE.MOVIE_GENRE_CODE= MOVIE_GENRE.MOVIE_GENRE_CODE ORDER BY MOVIE_ID;
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MOVIE_ID ---------1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 GENRE --------------------Drama Action and Adventure Comedy Action and Adventure Action and Adventure Action and Adventure Drama Action and Adventure Action and Adventure Drama Romance Comedy Comedy Drama Drama Comedy Romance Drama Action and Adventure Foreign
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MOVIE_TITLE ------------------------Mystic River The Last Samurai Something's Gotta Give The Italian Job Kill Bill: Vol. 1 Pirates of the Caribbean: Big Fish Man on Fire Master and Commander: The Lost in Translation Two Weeks Notice 50 First Dates Matchstick Men Cold Mountain Road to Perdition The School of Rock 13 Going on 30 Monster The Day After Tomorrow Das Boot
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Using the full table name as a column quali er can be tedious and time consuming, especially when table names can be 30 or more characters long in modern DBMSs. That s why a provision for table name aliases was included in SQL. They work much like the column name aliases available in the SELECT clause, except that the keyword AS is not used (in most SQL implementations) just leave a space between the table name and its alias in the FROM list. While some people use mnemonics for table name aliases, it s more common to see sequential capital letters use (that is, A, B, C, and so forth). Once you assign an alias to a table name in the FROM clause, you must use the alias instead of the table name throughout the SQL statement. Table name aliases in the SELECT clause will seem odd at rst because you use the alias before you ve actually de ned it (the SELECT clause precedes the FROM clause), and you may nd it easier to ll in the FROM clause before you ll in the column list in the SELECT clause as you write SQL statements. The following example shows the statement we just looked at with aliases for the table names added. Although not necessary, table name aliases were also added to the column list in the SELECT and ORDER BY clauses so you can see how that looks.
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SELECT A.MOVIE_ID, B.MOVIE_GENRE_DESCRIPTION AS GENRE, A.MOVIE_TITLE FROM MOVIE A, MOVIE_GENRE B WHERE A.MOVIE_GENRE_CODE = B.MOVIE_GENRE_CODE ORDER BY A.MOVIE_ID;
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Joins Using the JOIN clause
As already mentioned, the JOIN clause is a newer addition to SQL, added in the SQL-92 standard, so the implementation you are using may not support it as yet, although most current SQL implementations do. The JOIN clause is written as a table reference in the FROM clause and essentially combines the table list in the FROM clause and the join conditions previously written in the WHERE clause into a single clause. Here is the general syntax of the JOIN clause for an inner join, followed by some examples.
table_name [INNER] JOIN table_name { ON condition | USING (column_name [,column_name]) }
Note the two options. The ON clause allows for speci cation of a condition just like the one in the WHERE clause in the prior example. The USING clause, on the other hand, simply lists the column names to be used for matching rows. However, the USING clause only works when the columns to be matched have identical names in both tables. Here are some examples: JOIN with the ON condition:
SELECT MOVIE_ID, MOVIE_GENRE_DESCRIPTION AS GENRE, MOVIE_TITLE FROM MOVIE JOIN MOVIE_GENRE ON MOVIE.MOVIE_GENRE_CODE = MOVIE_GENRE.MOVIE_GENRE_CODE ORDER BY MOVIE_ID;
JOIN with table aliases instead of table names:
SELECT MOVIE_ID, MOVIE_GENRE_DESCRIPTION AS GENRE, MOVIE_TITLE FROM MOVIE A JOIN MOVIE_GENRE B ON A.MOVIE_GENRE_CODE = B.MOVIE_GENRE_CODE ORDER BY MOVIE_ID;
JOIN with the USING keyword (instead of the ON condition). This is a very nice shorthand option when the columns in the two tables have the same name. However, it is a very new standard, so as of this writing, only Oracle and MySQL support this syntax (Microsoft SQL Server and DB2 do not).
SELECT MOVIE_ID, MOVIE_GENRE_DESCRIPTION AS GENRE, MOVIE_TITLE FROM MOVIE JOIN MOVIE_GENRE USING (MOVIE_GENRE_CODE) ORDER BY MOVIE_ID;
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