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through each row of the SALESREPS table, one by one, and applies the search condition to the row. When a column name appears in the search condition (such as the MANAGER column in this example), SQL uses the value of the column in the current row. For each row, the search condition can produce one of three results: If the search condition is TRUE, the row is included in the query results. For example, the row for Bill Adams has the correct MANAGER value and is included. If the search condition is FALSE, the row is excluded from the query results. For example, the row for Sue Smith has the wrong MANAGER value and is excluded. If the search condition has a NULL (unknown) value, the row is excluded from the query results. For example, the row for Sam Clark has a NULL value for the MANAGER column and is excluded. Figure 6-5 shows another way to think about the role of the search condition in the WHERE clause. Basically, the search condition acts as a filter for rows of the table. Rows that satisfy the search condition pass through the filter and become part of the query results. Rows that do not satisfy the search condition are trapped by the filter and excluded from the query results.
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SQL offers a rich set of search conditions that allows you to specify many different kinds of queries efficiently and naturally. Five basic search conditions (called predicates in the ANSI/ ISO standard) are summarized here and are described in the sections that follow: Comparison test Compares the value of one expression with the value of another expression. Use this test to select offices in the Eastern region, or salespeople whose sales are above their quotas. Range test Tests whether the value of an expression falls within a specified range of values. Use this test to find salespeople whose sales are between $100,000 and $500,000. Set membership test Checks whether the value of an expression matches one of a set of values. Use this test to select offices located in New York, Chicago, or Los Angeles. Pattern matching test Checks whether the value of a column containing string data matches a specified pattern. Use this test to select customers whose names start with the letter E. Null value test Checks whether a column has a NULL (unknown) value. Use this test to find the salespeople who have not yet been assigned to a manager.
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PART II
The Comparison Test (=, <>, <, <=, >, >=)
The most common search condition used in a SQL query is a comparison test. In a comparison test, SQL computes and compares the values of two SQL expressions for each row of data. The expressions can be as simple as a column name or a constant, or they can be more complex arithmetic expressions. SQL offers six different ways of comparing the two expressions, as shown in Figure 6-6.
FIGURE 6-6
Comparison test syntax diagram
Part II:
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Some examples of typical comparison tests follow. Find salespeople hired before 2006.
SELECT NAME FROM SALESREPS WHERE HIRE_DATE < '2006-01-01'; NAME -----------Sue Smith Bob Smith Dan Roberts Paul Cruz
Note that SQL products do not handle dates in the same way, because vendors were pressed to support a date data type before the SQL standard was written. The YYYY-MM-DD format shown in the preceding example works for most SQL products, but you may have to change it for some products. For example, for Oracle, you either need to change the date to the default Oracle format ('01-JAN-88'), or you need to change the default date format for your session using the command ALTER SESSION SET NLS_DATE_FORMAT='YYYY-MM-DD'. List the offices whose sales fall below 80 percent of target.
SELECT CITY, SALES, TARGET FROM OFFICES WHERE SALES < (.8 * TARGET); CITY SALES TARGET ------- ------------ -----------Denver $186,042.00 $300,000.00
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