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Inner Joins in SQL2 *
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Figure 7-13 shows a simplified form of the extended SQL2 syntax for the FROM clause. It s easiest to understand all of the options provided by considering each type of join,
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Extended FROM clause in the SQL2 standard
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one by one, starting with the basic inner join and then moving to the various forms of outer join. The standard inner join of the GIRLS and BOYS tables can be expressed in SQL1 language:
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Multitable Queries (Joins)
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SELECT * FROM GIRLS, BOYS WHERE GIRLS.CITY = BOYS.CITY
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This is still an acceptable statement in SQL2. The writers of the SQL2 standard really couldn t have made it illegal without breaking all of the millions of multitable SQL queries that had already been written by the early 1990s. But the SQL2 standard specifies an alternative way of expressing the same query:
SELECT * FROM GIRLS INNER JOIN BOYS ON GIRLS.CITY = BOYS.CITY RETRIEVING DATA
Note that the two tables to be joined are explicitly connected by a JOIN operation, and the search condition that describes the join is now specified in an ON clause within the FROM clause. The search condition following the keyword ON can be any search condition that specifies the criteria used to match rows of the two joined tables. The columns referenced in the search condition must come only from the two joined tables. For example, assume that the BOYS table and the GIRLS table were each extended by adding an AGE column. Here is a join that matches girl/boy pairs in the same city and also requires that the boy and girl in each pair be the same age:
SELECT FROM ON AND * GIRLS INNER JOIN BOYS (GIRLS.CITY = BOYS.CITY) (GIRLS.AGE = BOYS.AGE)
In these simple two-table joins, the entire contents of the WHERE clause simply moved into the ON clause, and the ON clause doesn t add any functionality to the SQL language. However, recall from earlier in this chapter that in an outer join involving three tables or more, the order in which the joins occur affect the query results. The ON clause provides detailed control over how these multitable joins are processed, as described later in this chapter. The SQL2 standard permits another variation on the simple inner join query between the GIRLS and BOYS tables. Because the matching columns in the two tables have the same names and are being compared for equality (which is often the case), an alternative form of the ON clause, specifying a list of matching column names, can be used:
SELECT * FROM GIRLS INNER JOIN BOYS USING (CITY, AGE)
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The USING clause specifies a comma-separated list of the matching column names, which must be identical in both tables. It is completely equivalent to the ON clause that specifies each matching column pair explicitly, but it s a lot more compact and therefore easier to understand. Of course, if the matching columns have different names in the BOYS table and GIRLS table, then an ON clause or a WHERE clause with an equals test must be used. The ON clause must also be used if the join does not involve equality of the matching columns. For example, if you wanted to select girl/boy pairs where the girl was required to be older than the boy, you must use an ON clause to specify the join:
SELECT FROM ON AND * GIRLS INNER JOIN BOYS (GIRLS.CITY = BOYS.CITY GIRLS.AGE > BOYS.AGE)
There is one final variation on this simple query that illustrates another feature of the SQL2 FROM clause. A join between two tables where the matching columns are exactly those specific columns from the two tables that have identical names is called a natural join, because usually this is precisely the most natural way to join the two tables. The query selecting girl/boy pairs who live in the same city and have the same age can be expressed as a natural join using this SQL2 query:
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