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RETRIEVAL: SOME ADVANCED FEATURES
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Correlated Subqueries
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SQL also supports correlated subqueries. Look at the example in Listing 9-6, and you will find out why these subqueries are referred to as being correlated. Listing 9-6. Correlated Subquery Example select e.ename, e.init, from employees e where e.msal > (select from where ); ENAME -------ALLEN JONES BLAKE SCOTT KING FORD INIT MSAL ----- -------JAM 1600 JM 2975 R 2850 SCJ 3000 CC 5000 MG 3000 e.msal avg(x.msal) employees x x.deptno = e.deptno
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-- Note the reference to e
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You might want to compare this query with Listing 8-30 in the previous chapter, because they are similar. This query shows all employees who earn a higher salary than the average salary of their own department. There is one thing that makes this subquery special: it contains a reference to the tuple variable e (see e.DEPTNO in the fifth line) from the main query. This means that you cannot execute this subquery independently, in isolation, because that would result in an Oracle error message. You must interpret this subquery within the context of a specific row from the main query. The subquery is related to the main query, thus the term correlated. The Oracle DBMS processes the query in Listing 9-6 as follows: The tuple variable e ranges over the EMPLOYEES table, thus assuming 14 different values. For each row e, the subquery is executed after replacing e.DEPTNO by the literal department value of row e.
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Caution Re-executing a subquery for every single row of the main query may have a significant performance impact. The Oracle optimizer will try to produce an efficient execution plan, and there are some smart optimization algorithms for correlated subqueries; nevertheless, it is always a good idea to consider and test performance while writing SQL statements for production systems. With many queries, performance with small data sets (such as you would find in a development databaes) is quite good, but they do not perform well when large, near-production size data sets are queried.
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In mathematics, a distinction is made between free and bound variables. In the subquery of Listing 9-6, x is the free variable and e is bound by the main query. Let s look at another example in Listing 9-7. This query provides the fourth youngest employee of the company or, to be more precise, all employees for which there are three younger colleagues. Note that the result isn t necessarily a set containing a single employee. Listing 9-7. Another Example of a Correlated Subquery select e.* from employees e where (select count(*) from employees x where x.bdate > e.bdate) = 3; EMPNO ENAME INIT JOB MGR BDATE MSAL COMM DEPTNO ------ -------- ----- -------- ------ ----------- ------ ------ -----7876 ADAMS AA TRAINER 7788 30-DEC-1966 1100 20 You can also formulate these types of queries using analytical functions, as described in Section 9.6 of this chapter.
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Correlated subqueries often occur in combination with the EXISTS operator. Again, let s start with an example. The query in Listing 9-8 shows all course offerings without registrations. Listing 9-8. Correlated Subquery with EXISTS Operator select o.* from offerings o where not exists (select r.* from registrations r where r.course = o.course and r.begindate = o.begindate); COURSE -----ERM PRO RSD XML BEGINDATE TRAINER LOCATION ----------- -------- -------15-JAN-2001 19-FEB-2001 DALLAS 24-FEB-2001 7788 CHICAGO 18-SEP-2000 BOSTON
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The EXISTS operator is not interested in the actual rows (and column values) resulting from the subquery, if any. This operator checks for only the existence of subquery results. If the subquery returns at least one resulting row, the EXISTS operator evaluates to TRUE. If the subquery returns no rows at all, the result is FALSE.
RETRIEVAL: SOME ADVANCED FEATURES
Subqueries Following an EXISTS Operator
You could say that the EXISTS and NOT EXISTS operators are kind of empty set checkers. This implies that it doesn t matter which expressions you specify in the SELECT clause of the subquery. For example, you could also have written the query of Listing 9-8 as follows: select o.* from offerings o where not exists (select 'x' from registrations r ...
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