free barcode generator using vb.net Figure 6-12: WHERE clause syntax diagram in Software

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Figure 6-12: WHERE clause syntax diagram
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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 NAME, QUOTA, SALES FROM SALESREPS WHERE SALES < QUOTA OR SALES < 300000.00 NAME -------------Sam Clark Bob Smith Tom Snyder Paul Cruz Nancy Angelli QUOTA ----------$275,000.00 $200,000.00 NULL $275,000.00 $300,000.00 SALES ----------$299,912.00 $142,594.00 $75,985.00 $286,775.00 $186,042.00
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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 NAME, QUOTA, SALES FROM SALESREPS WHERE SALES < QUOTA AND SALES < 300000.00 NAME -------------Bob Smith Nancy Angelli QUOTA ----------$200,000.00 $300,000.00 SALES ----------$142,594.00 $186,042.00
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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 NAME, QUOTA, SALES FROM SALESREPS WHERE SALES < QUOTA AND NOT SALES < 150000.00 NAME -------------QUOTA ----------SALES -----------
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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 NAME FROM SALESREPS WHERE (REP_OFFICE IN (22, 11, 12)) OR (MANAGER IS NULL AND HIRE_DATE >= '01-JUN-88') OR (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. Table 6-1: AND Truth Table
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TRUE
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FALSE
NULL
TRUE FALSE NULL
TRUE FALSE NULL
FALSE FALSE FALSE
NULL FALSE NULL
Table 6-2: OR Truth Table
TRUE
FALSE
NULL
TRUE FALSE NULL
TRUE TRUE TRUE
TRUE FALSE NULL
TRUE NULL NULL
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Table 6-3: NOT Truth Table
TRUE
FALSE
NULL
FALSE
TRUE
NULL
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. The SQL2 standard adds another logical search condition, the IS test, to the logic provided by AND, OR, and NOT. Figure 6-13 shows the syntax of the IS test, which checks to see whether the logical value of an expression or comparison test is TRUE, FALSE, or UNKNOWN (NULL).
Figure 6-13: IS test syntax diagram
For example, the IS test: ((SALES - QUOTA) > 10000.00) IS UNKNOWN can be used to find rows where the comparison cannot be done because either SALES or QUOTA has a NULL value. Similarly, the IS test: ((SALES - QUOTA) > 10000.00) IS FALSE will select rows where SALES are not significantly above QUOTA. As this example shows, the IS test doesn't really add to the expressive power of SQL, since the test could just as easily have been written: NOT ((SALES - QUOTA) > 10000.00) For maximum portability, it's a good idea to avoid the tests and write the expressions using only AND, OR, and NOT. It's not always possible to avoid the IS UNKNOWN form of the test.
Sorting Query Results (ORDER BY Clause)
Like the rows of a table in the database, the rows of query results are not arranged in any particular order. You can ask SQL to sort the results of a query by including the ORDER BY clause in the SELECT statement. The ORDER BY clause, shown in Figure 6-14, consists of the keywords ORDER BY, followed by a list of sort specifications separated by
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commas. For example, the results of this query are sorted on two columns, REGION and CITY:
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