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SELECT * FROM GIRLS NATURAL INNER JOIN BOYS
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If the NATURAL keyword is specified, the ON and USING clauses may not be used in the join specification, because the natural join specifically defines the search condition to be used to join the tables all of the columns with identical column names in both tables are to be matched. The SQL2 standard assumes that the default join between two tables is an inner join. You can omit the keyword INNER from any of the preceding examples, and the resulting query remains a legal SQL2 statement with the same meaning.
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Outer Joins in SQL2 *
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The SQL2 standard provides complete support for outer joins using the same clauses described in the preceding section for inner joins and additional keywords. For example, the full outer join of the GIRLS and BOYS tables (without the AGE columns) is generated by this query:
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SELECT * FROM GIRLS FULL OUTER JOIN BOYS ON GIRLS.CITY = BOYS.CITY
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As explained earlier in this chapter, the query results will contain a row for each matched girl/boy pair, as well as one row for each unmatched boy, extended with NULL values in the columns from the other, unmatched table. SQL2 allows the same variations for outer joins as for inner joins; the query could also have been written:
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SELECT * FROM GIRLS NATURAL FULL OUTER JOIN BOYS
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Just as the keyword INNER is optional in the SQL2 language, the SQL2 standard also allows you to omit the keyword OUTER. The preceding query could also have been written:
SELECT * FROM GIRLS FULL JOIN BOYS USING (CITY)
The DBMS can infer from the word FULL that an outer join is required. By specifying LEFT or RIGHT instead of FULL, the SQL2 language extends quite naturally to left or right outer joins. Here is the left outer join version of the same query:
SELECT * FROM GIRLS LEFT OUTER JOIN BOYS USING (CITY)
As described earlier in the chapter, the query results will include matched girl/boy pairs and NULL-extended rows for each unmatched row in the GIRLS table (the left table in the join), but the results do not include unmatched rows from the BOYS table. Conversely, the right outer join version of the same query, specified like this:
SELECT * FROM GIRLS RIGHT OUTER JOIN BOYS USING (CITY)
includes boy/girl pairs and unmatched rows in the BOYS table (the right table in the join) but does not include unmatched rows from the GIRLS table.
SQL: The Complete Reference
Cross Joins and Union Joins in SQL2 *
The SQL2 support for extended joins includes two other methods for combining data from two tables. A cross join is another name for the Cartesian product of two tables, as described earlier in this chapter. A union join is closely related to the full outer join; its query results are a subset of those generated by the full outer join. Here is a SQL2 query that generates the complete product of the GIRLS and BOYS tables:
SELECT * FROM GIRLS CROSS JOIN BOYS
By definition, the Cartesian product (also sometimes called the cross product, hence the name CROSS JOIN ) contains every possible pair of rows from the two tables. It multiplies the two tables, turning tables of, for example, three girls and two boys into a table of six (3 2 = 6) boy/girl pairs. No matching columns or selection criteria are associated with the cross products, so the ON clause and the USING clause are not allowed. Note that the cross join really doesn t add any new capabilities to the SQL language. Exactly the same query results can be generated with an inner join that specifies no matching columns. So the preceding query could just as well have been written as:
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