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Row Selection (WHERE Clause)
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SQL queries that retrieve all rows of a table are useful for database browsing and reports, but for little else. Usually you ll want to select only some of the rows in a table and include only these rows in the query results. The WHERE clause is used to specify the rows you want to retrieve. Here are some examples of simple queries that use the WHERE clause: Show me the offices where sales exceed target.
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RETRIEVING DATA SELECT CITY, SALES, TARGET FROM OFFICES WHERE SALES > TARGET CITY SALES TARGET ------------ ------------ -----------New York $692,637.00 $575,000.00 Atlanta $367,911.00 $350,000.00 Los Angeles $835,915.00 $725,000.00
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Show me the name, sales, and quota of employee number 105.
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SELECT NAME, SALES, QUOTA FROM SALESREPS WHERE EMPL_NUM = 105 NAME SALES QUOTA ----------- ------------ -----------Bill Adams $367,911.00 $350,000.00
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Show me the employees managed by Bob Smith (employee 104).
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SELECT NAME, SALES FROM SALESREPS WHERE MANAGER = 104 NAME SALES ------------ -----------Bill Adams $367,911.00 Dan Roberts $305,673.00 Paul Cruz $286,775.00
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The WHERE clause consists of the keyword WHERE followed by a search condition that specifies the rows to be retrieved. In the previous query, for example, the search condition is MANAGER = 104. Figure 6-5 shows how the WHERE clause works. Conceptually, SQL goes 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: I 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. I 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. I 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 6-6 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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Search Conditions
SQL offers a rich set of search conditions that allow 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:
Figure 6-5.
Row selection with the WHERE clause
6:
Simple Queries
RETRIEVING DATA
Figure 6-6.
The WHERE clause as a filter
I Comparison test. Compares the value of one expression to the value of another expression. Use this test to select offices in the Eastern region, or salespeople whose sales are above their quotas. I 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. I 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. I 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. I 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.
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 6-7. Next are some examples of typical comparison tests.
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