free barcode generator using vb.net Subqueries in the WHERE Clause in Software

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Subqueries in the WHERE Clause
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Subqueries are most frequently used in the WHERE clause of a SQL statement. When a subquery appears in the WHERE clause, it works as part of the row selection process. The very simplest subqueries appear within a search condition and produce a value that is used to test the search condition. Here is an example of a simple subquery: List the salespeople whose quota is less than 10% of the company-wide sales target. SELECT NAME FROM SALESREPS WHERE QUOTA < (.1 * (SELECT SUM(TARGET) FROM OFFICES)) NAME --------Bob Smith In this case, the subquery calculates the sum of the sales targets for all of the offices to determine the company-wide target, which is multiplied by 10 percent to determine the "cutoff" sales quota for the query. That value is then used in the search condition to check each row of the SALESREPS table and find the requested names. In this simple case, the subquery produces the same value for every row of the SALESREPS table; the QUOTA value for each salesperson is compared to the same company-wide number. In fact, you could carry out this query by first performing the subquery, to calculate the cutoff quota amount ($275,000 in the sample database), and then carry out the main query using this number in a simple WHERE clause: WHERE QUOTA < 275000 It's more convenient to use the subquery, but it's not essential. Usually subqueries are not this simple. For example, consider once again the query from the previous section: List the offices where the sales target for the office exceeds the sum of the individual salespeople's quotas. SELECT CITY FROM OFFICES WHERE TARGET > (SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = OFFICE) CITY ----------Chicago Los Angeles In this (more typical) case, the subquery cannot be calculated once for the entire query. The subquery produces a different value for each office, based on the quotas of the salespeople in that particular office. Figure 9-2 shows conceptually how SQL carries out the query. The main query draws its data from the OFFICES table, and the WHERE clause selects which offices will be included in the query results. SQL goes through the rows of the OFFICES table one-by-one, applying the test stated in the WHERE clause. The WHERE clause compares the value of the TARGET column in the current row to the value produced by the subquery. To test the TARGET value, SQL carries out the subquery, finding the sum of the quotas for salespeople in the "current" office. The subquery produces a single number, and the WHERE clause compares the number to the TARGET
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value, selecting or rejecting the current office based on the comparison. As the figure shows, SQL carries out the subquery repeatedly, once for each row tested by the WHERE clause of the main query.
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Figure 9-2: Subquery operation in the WHERE clause
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Outer References
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Within the body of a subquery, it's often necessary to refer to the value of a column in the "current" row of the main query. Consider once again the query from the previous sections: List the offices where the sales target for the office exceeds the sum of the individual salespeople's quotas. SELECT CITY FROM OFFICES WHERE TARGET > (SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = OFFICE) The role of the subquery in this SELECT statement is to calculate the total quota for those salespeople who work in a particular office specifically, the office currently being tested by the WHERE clause of the main query. The subquery does this by scanning the SALESREPS table. But notice that the OFFICE column in the WHERE clause of the subquery doesn't refer to a column of the SALESREPS table; it refers to a column of the OFFICES table, which is a part of the main query. As SQL moves through each row of the OFFICES table, it uses the OFFICE value from the current row when it carries out the subquery. The OFFICE column in this subquery is an example of an outer reference. An outer reference is a column name that does not refer to any of the tables named in the FROM clause of the subquery in which the column name appears. Instead, the column name refers to a column of a table specified in the FROM clause of the main query. As the previous example shows, when the DBMS examines the search condition in the subquery, the value of the column in an outer reference is taken from the row currently being tested by the main query.
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