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Subqueries and Query Expressions
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The subquery:
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SELECT MGR FROM OFFICES
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obviously produces the employee numbers of the managers, and therefore the query seems to be saying: Find each salesperson who is not the manager of any office.
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RETRIEVING DATA
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But that s not what the query says! What it does say is this: Find each salesperson who, for some office, is not the manager of that office. Of course for any given salesperson, it s possible to find some office where that salesperson is not the manager. The query results would include all the salespeople and therefore fail to answer the question that was posed! The correct query is:
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SELECT NAME, AGE FROM SALESREPS WHERE NOT (EMPL_NUM = ANY (SELECT MGR FROM OFFICES)) NAME AGE -------------- ---Mary Jones 31 Sue Smith 48 Dan Roberts 45 Tom Snyder 41 Paul Cruz 29 Nancy Angelli 49
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You can always turn a query with an ANY test into a query with an EXISTS test by moving the comparison inside the search condition of the subquery. This is usually a very good idea because it eliminates errors like the one just described. Here is an alternative form of the query, using the EXISTS test:
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SELECT NAME, AGE FROM SALESREPS WHERE NOT EXISTS (SELECT * FROM OFFICES WHERE EMPL_NUM = MGR)
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NAME AGE -------------- ---Mary Jones 31 Sue Smith 48 Dan Roberts 45 Tom Snyder 41 Paul Cruz 29 Nancy Angelli 49
The ALL Test *
Like the ANY test, the ALL test is used in conjunction with one of the six SQL comparison operators (=, <>, <, <=, >, >=) to compare a single test value to a column of data values produced by a subquery. To perform the test, SQL uses the specified comparison operator to compare the test value to each data value in the column, one at a time. If all of the individual comparisons yield a TRUE result, the ALL test returns a TRUE result. Here is an example of a request that can be handled with the ALL test: List the offices and their targets where all of the salespeople have sales that exceed 50 percent of the office s target.
SELECT CITY, TARGET FROM OFFICES WHERE (.50 * TARGET) < ALL (SELECT SALES FROM SALESREPS WHERE REP_OFFICE = OFFICE) CITY TARGET ------------ -----------Denver $300,000.00 New York $575,000.00 Atlanta $350,000.00
Conceptually, the main query tests each row of the OFFICES table, one by one. The subquery finds all of the salespeople who work in the current office and returns a column containing the sales for each salesperson. The WHERE clause of the main query then computes 50 percent of the office s target and uses it as a test value, comparing it to every sales value produced by the subquery. If all of the sales values exceed the calculated test value, the ALL test returns TRUE, and the office is included in the query results. If not, the office is not included in the query results. Like the ANY test, the ALL test can be difficult to understand because it involves an entire set of comparisons, not just one. Again, it helps if you read the test in a slightly different way than it appears in the statement. If this ALL test appears:
9:
Subqueries and Query Expressions
WHERE X < ALL (SELECT Y )
instead of reading it like this:
"where X is less than all select Y "
try reading the test like this:
"where, for all Y, X is less than Y"
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When you use this trick, the preceding query becomes: Select the offices where, for all salespeople who work in the office, 50 percent of the office s target is less than the salesperson s sales. If the subquery in an ALL test produces no rows of query results, or if the query results include NULL values, the operation of the ALL test may vary from one DBMS to another. The ANSI/ISO SQL standard specifies these detailed rules describing the results of the ALL test when the test value is compared to the column of subquery results: I If the subquery produces an empty column of query results, the ALL test returns TRUE. The comparison test does hold for every value produced by the subquery; there just aren t any values. I If the comparison test is TRUE for every data value in the column, then the ALL search condition returns TRUE. Again, the comparison test holds true for every value produced by the subquery. I If the comparison test is FALSE for any data value in the column, then the ALL search condition returns FALSE. In this case, you can conclusively state that the comparison test does not hold true for every data value produced by the query. I If the comparison test is not FALSE for any data value in the column, but it is NULL for one or more of the data values, then the ALL search condition returns NULL. In this situation, you cannot conclusively state whether there is a value produced by the subquery for which the comparison test does not hold true; there may or may not be, depending on the actual (but currently unknown) values for the NULL data. The subtle errors that can occur when the ANY test is combined with the inequality (<>) comparison operator also occur with the ALL test. As with the ANY test, the ALL test can always be converted into an equivalent EXISTS test by moving the comparison inside the subquery.
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