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NULL value test (IS NULL) syntax diagram
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This query uses the NULL value test to find the salesperson in the sample database who has not yet been assigned to an office: Find the salesperson not yet assigned to an office.
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SELECT NAME FROM SALESREPS WHERE REP_OFFICE IS NULL NAME ----------Tom Snyder
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RETRIEVING DATA
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The negated form of the NULL value test (IS NOT NULL) finds rows that do not contain a NULL value: List the salespeople who have been assigned to an office.
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SELECT NAME FROM SALESREPS WHERE REP_OFFICE IS NOT NULL NAME -------------Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Larry Fitch Paul Cruz Nancy Angelli
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Unlike the previously described search conditions, the NULL value test cannot yield a NULL result. It is always either TRUE or FALSE.
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SQL: The Complete Reference
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It may seem strange that you can t just test for a NULL value using a simple comparison search condition, such as this:
SELECT NAME FROM SALESREPS WHERE REP_OFFICE = NULL
The NULL keyword can t be used here because it isn t really a value; it s just a signal that the value is unknown. Even if the comparison test:
REP_OFFICE = NULL
were legal, the rules for handling NULL values in comparisons would cause it to behave differently from what you might expect. When SQL encountered a row where the REP_ OFFICE column was NULL, the search condition would test:
NULL = NULL
Is the result TRUE or FALSE Because the values on both sides of the equal sign are unknown, SQL can t tell, so the rules of SQL logic say that the search condition itself must yield a NULL result. Because the search condition doesn t produce a true result, the row is excluded from the query results precisely the opposite of what you wanted to happen! As a result of the way SQL handles NULLs in comparisons, you must explicitly use the NULL value test to check for NULL values.
Compound Search Conditions (AND, OR, and NOT)
The simple search conditions described in the preceding sections return a value of TRUE, FALSE, or NULL when applied to a row of data. Using the rules of logic, you can combine these simple SQL search conditions to form more complex ones, as shown in Figure 6-12. Note that the search conditions combined with AND, OR, and NOT may themselves be compound search conditions.
Figure 6-12.
WHERE clause syntax diagram
6:
Simple Queries
The keyword OR is used to combine two search conditions when one or the other (or both) must be true: Find salespeople who are under quota or with sales under $300,000.
SELECT FROM WHERE OR NAME, QUOTA, SALES SALESREPS SALES < QUOTA SALES < 300000.00 RETRIEVING DATA
NAME QUOTA SALES -------------- ------------ -----------Sam Clark $275,000.00 $299,912.00 Bob Smith $200,000.00 $142,594.00 Tom Snyder NULL $75,985.00 Paul Cruz $275,000.00 $286,775.00 Nancy Angelli $300,000.00 $186,042.00
You can also use the keyword AND to combine two search conditions that must both be true: Find salespeople who are under quota and with sales under $300,000.
SELECT FROM WHERE AND NAME, QUOTA, SALES SALESREPS SALES < QUOTA SALES < 300000.00
NAME QUOTA SALES -------------- ------------ -----------Bob Smith $200,000.00 $142,594.00 Nancy Angelli $300,000.00 $186,042.00
Finally, you can use the keyword NOT to select rows where a search condition is false: Find all salespeople who are under quota, but whose sales are not under $150,000.
SELECT FROM WHERE AND NAME, QUOTA, SALES SALESREPS SALES < QUOTA NOT SALES < 150000.00
SQL: The Complete Reference
NAME QUOTA SALES -------------- ------------ -----------Nancy Angelli $300,000.00 $186,042.00
Using the logical AND, OR, and NOT keywords and parentheses to group the search criteria, you can build very complex search criteria, such as the one in this query: Find all salespeople who either: (a) work in Denver, New York, or Chicago; or (b) have no manager and were hired since June 1988; or (c) are over quota, but have sales of $600,000 or less.
SELECT FROM WHERE OR OR NAME SALESREPS (REP_OFFICE IN (22, 11, 12)) (MANAGER IS NULL AND HIRE_DATE >= '01-JUN-88') (SALES > QUOTA AND NOT SALES > 600000.00)
Exactly why you might want to see this particular list of names is a mystery, but the example does illustrate a reasonably complex query. As with simple search conditions, NULL values influence the outcome of compound search conditions, and the results are subtle. In particular, the result of (NULL OR TRUE) is TRUE, not NULL, as you might expect. Tables 6-1, 6-2, and 6-3 specify truth tables for AND, OR, and NOT, respectively, and show the impact of NULL values. When more than two search conditions are combined with AND, OR, and NOT, the ANSI/ISO standard specifies that NOT has the highest precedence, followed by AND and then OR. To ensure portability, it s always a good idea to use parentheses and remove any possible ambiguity.
TRUE FALSE NULL Table 6-1.
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