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PART II
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Inner Joins in Standard SQL
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Figure 7-13 shows a simplified form of the extended standard SQL syntax for the FROM clause. It s easiest to understand all of the options provided by considering each type of join, 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, expressed in the original SQL notation:
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SELECT * FROM GIRLS, BOYS WHERE GIRLS.CITY = BOYS.CITY;
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is still an acceptable statement in the latest version of the standard. The standard writers 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 modern SQL standard allows these alternative ways of expressing an inner join, which we have already seen in earlier examples:
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SELECT * FROM GIRLS INNER JOIN BOYS ON GIRLS.CITY = BOYS.CITY; SELECT * FROM GIRLS INNER JOIN BOYS USING (CITY); SELECT * FROM GIRLS NATURAL INNER JOIN BOYS;
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The INNER keyword is optional; an inner join is the default. The NATURAL JOIN form of the statement can be used if all of the identically named columns in the two tables are matching columns; otherwise, the USING clause must be used to indicate specific matching columns. In this case, the matching columns are NAME and CITY, and since none of the boys has the same name as one of the girls, that NATURAL JOIN form of the query returns no rows. If the matching columns do not have identical names in the two tables, or if a nonequi-join is needed, then the full ON clause or WHERE clause must be used to specify the matching column conditions. The ON and WHERE clauses are also more widely supported than the NATURAL and USING variations.
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Part II:
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Retrieving Data
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FROM
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table-specification , natural-join-expression join-expression cross-product-expression union-expression
natural-join expression: table1 NATURAL INNER FULL LEFT RIGHT join expression: table1 INNER FULL LEFT RIGHT cross-product expression: table1 CROSS JOIN table2 JOIN table2 ON sends condition USING (column list) OUTER JOIN table2
OUTER
union-expression: table1 UNION JOIN table2
FIGURE 7-13
Extended FROM clause in the SQL standard
Outer Joins in Standard SQL*
We have already seen how the expanded SQL standard supports outer joins, such as the full, left, and right outer joins specified by these queries:
SELECT * FROM GIRLS FULL OUTER JOIN BOYS ON GIRLS.CITY = BOYS.CITY; SELECT * FROM GIRLS LEFT OUTER JOIN BOYS ON GIRLS.CITY = BOYS.CITY;
7:
Multitable Queries (Joins)
SELECT * FROM GIRLS RIGHT OUTER JOIN BOYS ON GIRLS.CITY = BOYS.CITY;
The use of the OUTER keyword is optional; the DBMS can infer from the keyword FULL, LEFT, or RIGHT that an outer join is required. The results of the examples shown will all be different the FULL OUTER JOIN will return all the rows from both tables, the LEFT OUTER JOIN will return all the rows from the left (GIRLS) table plus matching rows from the right table (BOYS), and the RIGHT OUTER JOIN will return all the rows from the right (BOYS) table plus matching rows from the left (GIRLS) table. As for INNER joins, a natural join can be specified with the NATURAL keyword, eliminating the need to explicitly name the matching columns. Similarly, the matching columns can be named in a USING clause.
PART II
Cross Joins in Standard SQL*
The 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. Here is a 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
SELECT * FROM GIRLS, BOYS;
The use of the keywords CROSS JOIN in the FROM clause simply makes the cross join more explicit. In most databases, the cross join of two tables by itself is of very little practical use. Its usefulness really comes as a building block for more complex query expressions that start with the cross product of two tables and then use summary query capabilities (described in the next chapter) or set operations to further manipulate the results. At this writing, DB2 does not support the cross join syntax, but the same effect can be achieved with the older SQL syntax. The union join combines some of the features of the UNION operation (described in the previous chapter) with some of the features of the join operations described in this chapter. However, the UNION JOIN was deprecated in the SQL:1999 standard and removed entirely from the SQL:2003 standard. So, if you are using a DBMS that supports a newer version of the standard, it s likely that it has no support for the UNION JOIN syntax. In fact, none of the current versions of Oracle, SQL Server, MySQL, and DB2 supports it.
Part II:
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