barcode generator in vb.net 2005 Retrieving Data in Software

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Retrieving Data
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The escape character is specified as a one-character constant string in the ESCAPE clause of the search condition, as shown in Figure 6-9. Here is an example using a dollar sign ($) as the escape character: Find products whose product IDs start with the four letters A%BC .
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SELECT ORDER_NUM, PRODUCT FROM ORDERS WHERE PRODUCT LIKE 'A$%BC%' ESCAPE '$';
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The first percent sign in the pattern, which follows an escape character, is treated as a literal percent sign; the second functions as a wildcard. The use of escape characters is very common in pattern matching applications, which is why the ANSI/ISO standard specified it. However, it was not a part of the early SQL implementations, and some database systems have implemented other pattern-matching schemes. To ensure portability, the ESCAPE clause should be avoided.
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The Null Value Test (IS NULL)
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NULL values create a three-valued logic for SQL search conditions. For any given row, the result of a search condition may be TRUE or FALSE, or it may be NULL because one of the columns used in evaluating the search condition contains a NULL value. Sometimes it s useful to check explicitly for NULL values in a search condition and handle them directly. SQL provides a special NULL value test (IS NULL), shown in Figure 6-10, to handle this task. 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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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;
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FIGURE 6-10
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NULL value test (IS NULL) syntax diagram
6:
Simple Queries
NAME -------------Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Larry Fitch Paul Cruz Nancy Angelli
Unlike the previously described search conditions, the NULL value test cannot yield a NULL result. It is always either TRUE or FALSE. 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;
PART II
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 6-11.
FIGURE 6-11
WHERE clause syntax diagram
Part II:
Retrieving Data
Note that the search conditions combined with AND, OR, and NOT may themselves be compound search conditions. 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;
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;
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 (a) work in Denver, New York, or Chicago; or (b) have no manager and were hired since June 2006; or (c) are over quota, but have sales of $600,000 or less.
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