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Single-Row Retrieval
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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 -----------------Ace International CREDIT_LIMIT -----------$35,000.00
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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.
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NULL Value Considerations
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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
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that the results of these two queries: 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 would include every row of the SALESREPS table, but 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 TRUE, FALSE, or NULL result. Only rows where the search condition yields a TRUE result are included in the query results.
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Range Test (BETWEEN)
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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.
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Figure 6-8: Range test (BETWEEN) syntax diagram
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This example shows a typical range test:
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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 ORDER_DATE MFR PRODUCT ---------- --------- -----------112961 17-DEC-89 REI 2A44L 112968 12-OCT-89 ACI 41004 112963 17-DEC-89 ACI 41004 112983 27-DEC-89 ACI 41004 112979 12-OCT-89 ACI 4100Z 112992 04-NOV-89 ACI 41002 112975 12-OCT-89 REI 2A44G 112987 31-DEC-89 ACI 4100Y AMOUNT ---------$31,500.00 $3,978.00 $3,276.00 $702.00 $15,000.00 $760.00 $2,100.00 $27,500.00
The BETWEEN test includes the endpoints of the range, so orders placed on October 1 or December 31 are included in the query results. Here is another example of a range test: Find the orders that fall into various amount ranges. SELECT ORDER_NUM, AMOUNT FROM ORDERS WHERE AMOUNT BETWEEN 20000.00 AND 29999.99 ORDER_NUM ---------113036 112987 113042 AMOUNT ---------$22,500.00 $27,500.00 $22,500.00
SELECT ORDER_NUM, AMOUNT FROM ORDERS WHERE AMOUNT BETWEEN 30000.00 AND 39999.99 ORDER_NUM ---------112961 113069 AMOUNT ---------$31,500.00 $31,350.00
SELECT ORDER_NUM, AMOUNT FROM ORDERS WHERE AMOUNT BETWEEN 40000.00 AND 49999.99 ORDER_NUM ---------113045 AMOUNT ---------$45,000.00
The negated version of the range test (NOT BETWEEN) checks for values that fall outside the range, as in this example:
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List salespeople whose sales are not between 80 percent and 120 percent of quota. SELECT NAME, SALES, QUOTA FROM SALESREPS WHERE SALES NOT BETWEEN (.8 * QUOTA) AND (1.2 * QUOTA) NAME -------------Mary Jones Sue Smith Bob Smith Nancy Angelli SALES ----------$392,725.00 $474,050.00 $142,594.00 $186,042.00 QUOTA ----------$300,000.00 $350,000.00 $200,000.00 $300,000.00
The test expression specified in the BETWEEN test can be any valid SQL expression, but in practice it's usually just a column name, as in the previous examples. The ANSI/ISO standard defines relatively complex rules for the handling of NULL values in the BETWEEN test: If the test expression produces a NULL value, or if both expressions defining the range produce NULL values, then the BETWEEN test returns a NULL result. If the expression defining the lower end of the range produces a NULL value, then the BETWEEN test returns FALSE if the test value is greater than the upper bound, and NULL otherwise. If the expression defining the upper end of the range produces a NULL value, then the BETWEEN test returns FALSE if the test value is less than the lower bound, and NULL otherwise. Before relying on this behavior, it's a good idea to experiment with your DBMS. It's worth noting that the BETWEEN test doesn't really add to the expressive power of SQL, because it can be expressed as two comparison tests. The range test: A BETWEEN B AND C is completely equivalent to: (A >= B) AND (A < = C) However, the BETWEEN test is a simpler way to express a search condition when you're thinking of it in terms of a range of values.
Set Membership Test (IN)
Another common search condition is the set membership test (IN), shown in Figure 6-9. It tests whether a data value matches one of a list of target values. Here are several queries that use the set membership test:
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