barcode generator in vb.net 2005 Subqueries and Query Expressions in Software

Generate QR in Software Subqueries and Query Expressions

Subqueries and Query Expressions
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FIGURE 9-5 Existence test (EXISTS) syntax diagram
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EXISTS NOT
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Conceptually, SQL processes this query by going through the PRODUCTS table and performing the subquery for each product. The subquery produces a column containing the order numbers of any orders for the current product that are over $25,000. If there are any such orders (that is, if the column is not empty), the EXISTS test is TRUE. If the subquery produces no rows, the EXISTS test is FALSE. The EXISTS test cannot produce a NULL value. You can reverse the logic of the EXISTS test using the NOT EXISTS form. In this case, the test is TRUE if the subquery produces no rows, and FALSE otherwise. Notice that the EXISTS search condition doesn t really use the results of the subquery at all. It merely tests to see whether the subquery produces any results. For this reason, SQL relaxes the rule that subqueries must return a single column of data and allows you to use the SELECT * form in the subquery of an EXISTS test. The previous subquery could thus have been written as: List the products for which an order of $25,000 or more has been received.
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SELECT DESCRIPTION FROM PRODUCTS WHERE EXISTS (SELECT FROM WHERE AND AND
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PART II
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* ORDERS PRODUCT = PRODUCT_ID MFR = MFR_ID AMOUNT >= 25000.00);
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In practice, the subquery in an EXISTS test is usually written using the SELECT * notation. Here are some additional examples of queries that use EXISTS: List any customers assigned to Sue Smith who have not placed an order for over $3000.
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SELECT COMPANY FROM CUSTOMERS WHERE CUST_REP = (SELECT FROM WHERE AND NOT EXISTS (SELECT FROM WHERE AND COMPANY ----------------Carter & Sons Fred Lewis Corp.
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EMPL_NUM SALESREPS NAME = 'Sue Smith') * ORDERS CUST = CUST_NUM AMOUNT > 3000.00);
Part II:
Retrieving Data
List the of ces where there is a salesperson whose quota represents more than 55 percent of the of ce s target.
SELECT CITY FROM OFFICES WHERE EXISTS (SELECT FROM WHERE AND CITY -------Denver Atlanta
* SALESREPS REP_OFFICE = OFFICE QUOTA > (.55 * TARGET));
Note that in each of these examples, the subquery includes an outer reference to a column of the table in the main query. In practice, the subquery in an EXISTS test will always contain an outer reference that links the subquery to the row currently being tested by the main query.
Quantified Tests (ANY and ALL)*
The subquery version of the IN test checks whether a data value is equal to some value in a column of subquery results. SQL provides two quantified tests, ANY and ALL, that extend this notion to other comparison operators, such as greater than (>) and less than (<). Both of these tests compare a data value with the column of data values produced by a subquery, as shown in Figure 9-6.
test-expression
= <> < <= > >=
ANY ALL
subquery
FIGURE 9-6
Quanti ed comparison tests (ANY and ALL) syntax diagrams
9:
Subqueries and Query Expressions
The ANY Test*
The ANY 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 any of the individual comparisons yields a TRUE result, the ANY test returns a TRUE result. Here is an example of a request that can be handled with the ANY test: List the salespeople who have taken an order that represents more than 10 percent of their quota.
SELECT NAME FROM SALESREPS WHERE (.1 * QUOTA) < ANY (SELECT AMOUNT FROM ORDERS WHERE REP = EMPL_NUM); NAME -------------Sam Clark Larry Fitch Nancy Angelli
PART II
Conceptually, the main query tests each row of the SALESREPS table, one by one. The subquery finds all of the orders taken by the current salesperson and returns a column containing the order amounts for those orders. The WHERE clause of the main query then computes 10 percent of the current salesperson s quota and uses it as a test value, comparing it with every order amount produced by the subquery. If any order amount exceeds the calculated test value, the ANY test returns TRUE, and the salesperson is included in the query results. If not, the salesperson is not included in the query results. The keyword SOME is an alternative for ANY specified by the ANSI/ISO SQL standard. Either keyword generally can be used, but some DBMS brands do not support SOME. The ANY test can sometimes be difficult to understand because it involves an entire set of comparisons, not just one. It helps if you read the test in a slightly different way than it appears in the statement. If this ANY test appears:
WHERE X < ANY (SELECT Y )
instead of reading the test like this:
"where X is less than any select Y "
try reading it like this:
"where, for some Y, X is less than Y"
When you use this trick, the preceding query becomes Select the salespeople where, for some order taken by the salesperson, 10 percent of the salesperson s quota is less than the order amount. If the subquery in an ANY test produces no rows of query results, or if the query results include NULL values, the operation of the ANY test may vary from one DBMS to another.
Part II:
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