barcode generator in vb.net 2005 Retrieving Data in Software

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Retrieving Data
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The ANSI/ISO SQL standard specifies these detailed rules describing the results of the ANY test when the test value is compared with the column of subquery results: If the subquery produces an empty column of query results, the ANY test returns FALSE no value is produced by the subquery for which the comparison test holds. If the comparison test is TRUE for at least one of the data values in the column, then the ANY search condition returns TRUE indeed some value is produced by the subquery for which the comparison test holds. If the comparison test is FALSE for every data value in the column, then the ANY search condition returns FALSE. In this case, you can conclusively state that no value is produced by the subquery for which the comparison test holds. If the comparison test is not TRUE for any data value in the column, but it is NULL (unknown) for one or more of the data values, then the ANY search condition returns NULL. In this situation, you cannot conclusively state whether a value is produced by the subquery for which the comparison test holds; it may or may not be, depending on the actual (but currently unknown) values for the NULL data. The ANY comparison operator can be very tricky to use in practice, especially in conjunction with the inequality (<>) comparison operator. Here is an example that shows the problem: List the names and ages of all the people in the sales force who do not manage an of ce. It s tempting to express this query as shown in this example:
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SELECT NAME, AGE FROM SALESREPS WHERE EMPL_NUM <> ANY (SELECT MGR FROM OFFICES);
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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 of ce. But that s not what the query says! What it does say is this: Find each salesperson who, for some of ce, is not the manager of that of ce. 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));
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9:
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Subqueries and Query Expressions
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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:
SELECT NAME, AGE FROM SALESREPS WHERE NOT EXISTS (SELECT * FROM OFFICES WHERE EMPL_NUM = MGR); NAME AGE -------------- ---Mary Jones 31 Sue Smith 48 Dan Roberts 45 Tom Snyder 41 Paul Cruz 29 Nancy Angelli 49
PART II
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 with a column of data values produced by a subquery. To perform the test, SQL uses the specified comparison operator to compare the test value with 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 of ces and their targets where all of the salespeople have sales that exceed 50 percent of the of ce 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
Part II:
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