barcode generator in vb.net 2005 Retrieving Data in Software

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Retrieving Data
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The subquery form of the comparison test uses one of the simple comparison operators to compare a test value with the single value returned by a subquery. The subquery form of the set membership test (IN) matches a test value to the set of values returned by a subquery. The existence test (EXISTS) checks whether a subquery returns any values. The quantified tests (ANY and ALL) use one of the simple comparison operators to compare a test value with all of the values returned by a subquery, checking to see whether the comparison holds for some or all of the values. A subquery may include an outer reference to a table in any of the queries that contains it, linking the subquery to the current row of that query. Figure 9-8 shows the final version of the rules for SQL query processing, extended to include subqueries. It provides a complete definition of the query results produced by a SELECT statement.
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To generate the query results for a SELECT statement: 1. 2. 3. If the statement is a UNION of SELECT statements, apply Steps 2 through 7 to each of the statements to generate their individual query results. Form the product of the tables named in the FROM clause. If the FROM clause names a single table, the product is that table. If there is a WHERE clause, apply its search condition to each row of the product table, retaining those rows for which the search condition is TRUE (and discarding those for which it is FALSE or NULL). If the WHERE clause contains a subquery, the subquery is performed for each row as it is tested. If there is a GROUP BY clause, arrange the remaining rows of the product table into row groups, so that the rows in each group have identical values in all of the grouping columns. If there is a HAVING clause, apply its search condition to each row group, retaining those groups for which the search condition is TRUE (and discarding those for which it is FALSE or NULL). If the HAVING clause contains a subquery, the subquery is performed for each row group as it is tested. For each remaining row (or row group), calculate the value of each item in the select list to produce a single row of query results. For a simple column reference, use the value of the column in the current row (or row group). For a column function, use the current row group as its argument if GROUP BY is specified; otherwise, use the entire set of rows. If SELECT DISTINCT is specified, eliminate any duplicate rows of query results that were produced. If the statement is a UNION of SELECT statements, merge the query results for the individual statements into a single table of query results. Eliminate duplicate rows unless UNION ALL is specified. If there is an ORDER BY clause, sort the query results as specified. The rows generated by this procedure comprise the query results.
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FIGURE 9-8
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SQL query processing rules ( nal version)
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Subqueries and Query Expressions
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Advanced Queries*
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The SQL queries described thus far in s 6 through 9 are the mainstream capabilities provided by most SQL implementations. The combination of features they represent column selection in the SELECT clause, row selection criteria in the WHERE clause, multitable joins in the FROM clause, summary queries in the GROUP BY and HAVING clauses, and subqueries for more complex requests give the user a powerful set of data retrieval and data analysis capabilities. However, database experts have pointed out many limitations of these mainstream query capabilities, including these: No decision making within queries Suppose you wanted to generate a twocolumn report from the sample database showing the name of each sales office and either its annual sales target or its year-to-date sales, whichever is larger. With the mainstream SQL query features, this is hard to do. Or suppose you had a database that kept track of sales by quarter (four columns of data for each office) and wanted to write a program that displayed offices and their sales for a specific (usersupplied) quarter. Again, this program is more difficult to write using mainstream SQL queries. You must include four separate SQL queries (one for each quarter), and the program logic must select which query to run, based on user input. This simple case isn t too difficult, but in a more general case, the program could become much more complex. Limited use of subqueries The simplest example of this limitation is the SQL1 restriction that a subquery can appear only on the right side of a comparison test in a WHERE clause. The database request List the offices where the sum of the salespeople s quotas is less than the office target. is most directly expressed as this query:
SELECT OFFICE FROM OFFICES WHERE (SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = OFFICE) < TARGET;
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