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RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
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EMPLOYEE -------ADAMS CLARK JONES JONES SMITH TURNER
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The syntax of Listing 8-8 is more elegant than the syntax in Listing 8-7, because the join is fully specified in the FROM clause and the WHERE clause contains only the filtering (i.e., the non-join) condition.
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You can also use the NATURAL JOIN operator in the FROM clause. Listing 8-9 shows an example that joins the EMPLOYEES table with the HISTORY table. Question: Before reading on, how is it possible that Listing 8-9 produces 15 rows in the result, instead of 14 Listing 8-9. Natural Join Example select ename, beginyear, msal, deptno from employees natural join history; ENAME BEGINYEAR MSAL DEPTNO -------- --------- -------- -------SMITH 2000 800 20 ALLEN 1999 1600 30 WARD 1992 1250 30 WARD 2000 1250 30 JONES 1999 2975 20 MARTIN 1999 1250 30 BLAKE 1989 2850 30 CLARK 1988 2450 10 SCOTT 2000 3000 20 KING 2000 5000 10 TURNER 2000 1500 30 ADAMS 2000 1100 20 JONES 2000 800 30 FORD 2000 3000 20 MILLER 2000 1300 10 Explanation: To understand what s happening in Listing 8-9, you must know how the NATURAL JOIN operator is defined in the SQL language. Listing 8-9 illustrates the behavior of the NATURAL JOIN operator: 1. The NATURAL JOIN operator determines which columns the two tables (EMPLOYEES and HISTORY) have in common. In this case, these are the three columns EMPNO, MSAL, and DEPTNO.
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It joins the two tables (using an equijoin) over all columns they have in common. It suppresses the duplicate columns resulting from the join operation in the previous step. This is why you don t get an error message about MSAL and DEPTNO in the SELECT clause being ambiguously defined. Finally, the NATURAL JOIN operator evaluates the remaining query clauses. In Listing 8-9, the only remaining clause is the SELECT clause. The final result shows the desired four columns.
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Apparently, every employee occurs only once in the result, except WARD. This means that this employee has been employed by the same department (30) for the same salary (1250) during two distinct periods of his career. This is a pure coincidence. If the query had returned 14 rows instead of 15, we would probably not have been triggered to investigate the query for correctness. Remember that some wrong queries may give correct results by accident. This example shows that you should be very careful when using the NATURAL JOIN operator. Probably the biggest danger is that a natural join may suddenly start producing strange and undesirable results if you add new columns to your tables, or you rename existing columns, thus accidentally creating matching column names.
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Caution Natural joins are safe only if you practice a very strict column-naming standard in your database designs.
Equijoins on Columns with the Same Name
SQL offers an alternative way to specify equijoins, allowing you to explicitly specify the columns you want to participate in the equijoin operation. As you saw in Listing 8-8, you can use the ON clause followed by fully specified join predicates. You can also use the USING clause, specifying column names instead of full predicates. See Listing 8-10 for an example. Listing 8-10. JOIN ... USING Example select e.ename, e.bdate , h.deptno, h.msal from employees e join history h using (empno) where e.job = 'ADMIN'; ENAME -------JONES MILLER MILLER MILLER MILLER BDATE DEPTNO MSAL ----------- -------- -------03-DEC-1969 30 800 23-JAN-1962 10 1275 23-JAN-1962 10 1280 23-JAN-1962 10 1290 23-JAN-1962 10 1300
RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
Note that you need tuple variables again, because you join over only the EMPNO column; the columns h.DEPTNO and e.DEPTNO are now different. Figure 8-3 shows the syntax diagram of the ANSI/ISO join syntax, including the NATURAL JOIN operator, the ON clause, and the USING clause.
Figure 8-3. ANSI/ISO join syntax diagram Note that you can also use a CROSS JOIN syntax. The result is identical to the effect of the comma operator in the FROM clause: the Cartesian product. The examples in the remainder of this book will show a mixture of old-fashioned joins (as introduced in Section 8.2) and the alternative ANSI/ISO SQL join syntax explained in this section.
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