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SQL: The Complete Reference
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Comparison test syntax diagram
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Find salespeople hired before 1988.
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SELECT NAME FROM SALESREPS WHERE HIRE_DATE < '01-JAN-88' NAME -----------Sue Smith Bob Smith Dan Roberts Paul Cruz
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List the offices whose sales fall below 80 percent of target.
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SELECT CITY, SALES, TARGET FROM OFFICES WHERE SALES < (.8 * TARGET) CITY SALES TARGET ------- ------------ -----------Denver $186,042.00 $300,000.00
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SELECT CITY, MGR FROM OFFICES WHERE MGR <> 108
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Simple Queries
CITY MGR --------- ---New York 106 Chicago 104 Atlanta 105
As shown in Figure 6-7, the inequality comparison test is written as A < > B according to the ANSI/ISO SQL specification. Several SQL implementations use alternate notations, such as A != B (used by SQL Server) and A =B (used by DB2 and SQL/DS). In some cases, these are alternative forms; in others, they are the only acceptable form of the inequality test. When SQL compares the values of the two expressions in the comparison test, three results can occur: I If the comparison is true, the test yields a TRUE result. I If the comparison is false, the test yields a FALSE result. I If either of the two expressions produces a NULL value, the comparison yields a NULL result.
RETRIEVING DATA
Single-Row Retrieval
The most common comparison test is one that checks whether a column s value is equal to some constant. When the column is a primary key, the test isolates a single row of the table, producing a single row of query results, as in this example: Retrieve the name and credit limit of customer number 2107.
SELECT COMPANY, CREDIT_LIMIT FROM CUSTOMERS WHERE CUST_NUM = 2107 COMPANY CREDIT_LIMIT ------------------ ------------Ace International $35,000.00
This type of query is the foundation of forms-based database retrieval programs. The user enters a customer number into the form, and the program uses the number to construct and execute a query. It then displays the retrieved data in the form. Note that the SQL statements for retrieving a specific customer by number, as in this example, and retrieving all customers with a certain characteristic (such as those with credit limits over $25,000) both have exactly the same form. These two types of queries (retrieval by primary key and retrieval based on a search of the data) would be very different operations in a nonrelational database. This uniformity of approach makes SQL much simpler to learn and use than earlier query languages.
SQL: The Complete Reference
NULL Value Considerations
The behavior of NULL values in comparison tests can reveal some obviously true notions about SQL queries to be, in fact, not necessarily true. For example, it would seem that the results of these two queries would include every row of the SALESREPS table: List salespeople who are over quota.
SELECT NAME FROM SALESREPS WHERE SALES > QUOTA NAME -----------Bill Adams Mary Jones Sue Smith Sam Clark Dan Roberts Larry Fitch Paul Cruz
List salespeople who are under or at quota.
SELECT NAME FROM SALESREPS WHERE SALES < = QUOTA NAME -------------Bob Smith Nancy Angelli
However, the queries produce seven and two rows, respectively, for a total of nine rows, while there are ten rows in the SALESREPS table. Tom Snyder s row has a NULL value in the QUOTA column because he has not yet been assigned a quota. This row is not listed by either query; it vanishes in the comparison test. As this example shows, you need to think about NULL value handling when you specify a search condition. In SQL s three-valued logic, a search condition can yield a
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Simple Queries
TRUE, FALSE, or NULL result. Only rows where the search condition yields a TRUE result are included in the query results.
The Range Test (BETWEEN)
SQL provides a different form of search condition with the range test (BETWEEN) shown in Figure 6-8. The range test checks whether a data value lies between two specified values. It involves three SQL expressions. The first expression defines the value to be tested; the second and third expressions define the low and high ends of the range to be checked. The data types of the three expressions must be comparable. This example shows a typical range test: Find orders placed in the last quarter of 1989.
SELECT ORDER_NUM, ORDER_DATE, MFR, PRODUCT, AMOUNT FROM ORDERS WHERE ORDER_DATE BETWEEN '01-OCT-89' AND '31-DEC-89' ORDER_NUM ---------112961 112968 112963 112983 112979 112992 112975 112987 ORDER_DATE ----------17-DEC-89 12-OCT-89 17-DEC-89 27-DEC-89 12-OCT-89 04-NOV-89 12-OCT-89 31-DEC-89 MFR ---REI ACI ACI ACI ACI ACI REI ACI PRODUCT AMOUNT -------- ----------2A44L $31,500.00 41004 $3,978.00 41004 $3,276.00 41004 $702.00 4100Z $15,000.00 41002 $760.00 2A44G $2,100.00 4100Y $27,500.00
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