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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 with 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:
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WHERE X < ALL (SELECT Y )
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instead of reading it like this:
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"where X is less than all select Y "
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try reading the test like this:
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"where, for all Y, X is less than Y"
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When you use this trick, the preceding query becomes Select the of ces where, for all salespeople who work in the of ce, 50 percent of the of ce 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 with the column of subquery results: 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. 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. 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. 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 a value is 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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9:
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Subqueries and Query Expressions
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Subqueries and Joins
You may have noticed as you read through this chapter that many of the queries that were written using subqueries could also have been written as multitable queries, or joins. This is often the case, and SQL allows you to write the query either way. This example illustrates the point: List the names and ages of salespeople who work in of ces in the Western region.
SELECT NAME, AGE FROM SALESREPS WHERE REP_OFFICE IN (SELECT OFFICE FROM OFFICES WHERE REGION = 'Western'); NAME AGE -------------- ---Sue Smith 48 Larry Fitch 62 Nancy Angelli 49
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