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Note The ANSI/ISO SQL standard defines * as being an arbitrary literal in this case.
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Subqueries that follow an EXISTS operator are often correlated. Think about this for a moment. If they are uncorrelated, their result is precisely the same for each row from the main query. There are only two possible outcomes: the EXISTS operator results in TRUE for all rows or FALSE for all rows. In other words, EXISTS followed by an uncorrelated subquery becomes an all or nothing operator.
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Caution A subquery returning a null value is not the same as a subquery returning nothing (that is, the empty set). This will be demonstrated later in this section.
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EXISTS, IN, or JOIN
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See Listing 9-9 for another EXISTS example, to finish this section. The query is intended to provide the personal details of all employees who ever taught an SQL course. Listing 9-9. Another Correlated Subquery with EXISTS Operator select e.* from employees e where exists (select from where and EMPNO -----7369 7902 ENAME -------SMITH FORD INIT ----N MG
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o.* offerings o o.course = 'SQL' o.trainer = e.empno); JOB MGR BDATE MSAL COMM DEPTNO -------- ------ ----------- -------- ------ -----TRAINER 7902 17-DEC-1965 800 20 TRAINER 7566 13-FEB-1959 3000 20
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This problem can also be solved with an IN operator, as shown in Listing 9-10. The query results are omitted. Listing 9-10. Alternative Formulation for Listing 9-9 select e.* from employees e where e.empno in (select o.trainer from offerings o where o.course = 'SQL') You can also use a join to solve the problem, as shown in Listing 9-11. This is probably the most obvious approach, although the choice between writing joins or subqueries is highly subjective. Some people think bottom up and prefer subqueries; others think top down and prefer to write joins. Listing 9-11. Another Alternative Formulation for Listing 9-9 select DISTINCT e.* from employees e join offerings o on e.empno = o.trainer where o.course = 'SQL' Notice the DISTINCT option in the SELECT clause. Investigate what happens if you remove the DISTINCT option in Listing 9-11. You ll find that the query result will consist of three rows, instead of two. So far, we have considered only subqueries in the WHERE clause. However, you can use subqueries in other SQL statement components, such as the SELECT and FROM clauses. In the next sections, we will look at subqueries in these other clauses. NULLs with EXISTS AND IN in subquery results often cause problems for people writing SQL for Oracle database systems, especially for those used to writing SQL for other database systems. Not only can nulls in subquery results cause confusion, but they can lead to incorrect results. There are several key concepts to keep in mind: NULL is not data, but rather a condition of data being unknown. NULL = NULL, NULL != NULL or NULL IN (NULL) always evaluates to UNKNOWN, which is neither TRUE nor FALSE. It is not possible to join two rows with NULLs in the join column.
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We illustrate our point about the trouble NULLs cause with EXISTS and IN queries with the reports in Listing 9-12. The queries behind the reports show two different ways to generate a list of managers. One approach uses IN; the other uses EXISTS. At face value, either approach works, and there seems to be no difference between them. Listing 9-12. Selecting all managers using IN or EXISTS select ename from employees where empno in (select mgr from employees);
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ENAME -------JONES BLAKE CLARK SCOTT KING FORD select e1.ename from employees e1 where exists (select e2.mgr from employees e2 where e1.empno = e2.mgr); ENAME -------JONES BLAKE CLARK SCOTT KING FORD As you see from Listing 9-12, the use of IN or EXISTS are equivalent in terms of results, though the actual operations are different. IN builds a list of values that are used for comparison wth empno. EXISTS executes the subquery for each empno and returns TRUE if the join finds a matching empno. However, the two queries return the same results only because NULLs are not involved. If there was a NULL empno, the EXISTS subquery would not return a record for that employee number, because a NULL empno value would not join with the NULL mgr value (NULL = NULL does not evaluate to TRUE). EXISTS answers the question, is this value present in the specified table column If that value is present (as indicated by at least one row being returned from the subquery), the answer is yes and the EXISTS expression evaluates to TRUE. As NULLs cannot be equated, joining a NULL mgr to a NULL empno does not return TRUE. Essentially, the query joins the inner and outer tables and returns the rows that match, one at a time. If the main query value does not have a match in the subquery (i.e., the join does not return at least one row), then the EXISTS evaluates to FALSE. IN answers the question, does the value exist anywhere in this list If one list value matches the external value, then the expression evaluates to TRUE. One way to think of an IN list expression is to rephrase it as a series of OR expressions. For example, the following: 1234 IN (1234, NULL) is equivalent to: 1234 = 1234 OR 1234 = NULL Each equality check can be evaluated separately and the result would be TRUE OR UNKNOWN. Reference the truth table in 4.10 (in 4). TRUE OR UNKNOWN is TRUE. Essentially, once you find a match, you can stop looking and ignore any previous NOT TRUE (FALSE or UNKNOWN) results. If the value does not match at least one value in the list, then the expression returns FALSE.
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