barcode generator in vb.net 2005 Built-In Functions in Software

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A number of useful built-in functions are specified in SQL standard, and most SQL implementations add many more. These facilities often provide data type conversion facilities. For example, DB2 s built-in MONTH() and YEAR() functions take a DATE or TIMESTAMP value as their input and return an integer that is the month or year portion of the value. This query lists the name and month of hire for each salesperson in the sample database:
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SELECT NAME, MONTH(HIRE_DATE) FROM SALESREPS;
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5:
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and this one lists all salespeople hired in 2006:
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SELECT NAME, MONTH(HIRE_DATE) FROM SALESREPS WHERE YEAR(HIRE_DATE) = 2006;
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Built-in functions are also often used for data reformatting. Oracle s built-in TO_CHAR() function, for example, takes a DATE data type and a format specification as its arguments and returns a string containing a formatted character string version of the date. (This same function is also capable of converting numeric values to formatted character strings.) In the results produced by this query:
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SELECT NAME, TO_CHAR(HIRE_DATE,'DAY MONTH DD, YYYY') FROM SALESREPS;
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the hire dates will all have the format Wednesday June 14, 2007 because of the built-in function. In general, a built-in function can be specified in a SQL expression anywhere that a constant of the same data type can be specified. The built-in functions supported by popular SQL dialects are too numerous to list here. The IBM DB2 SQL dialects include about two dozen built-in functions, Oracle supports a different set of about two dozen built-in functions, and SQL Server has several dozen. The SQL2 standard incorporated the most useful built-in functions from these implementations, in many cases with slightly different syntax. These functions are summarized in Table 5-6.
Function BIT_LENGTH (string) CAST (value AS data_type) CHAR_LENGTH (string) CONVERT (string USING conv) CURRENT_DATE CURRENT_TIME (precision) CURRENT_TIMESTAMP (precision) EXTRACT (part FROM source) LOWER (string) OCTET_LENGTH (string) POSITION (target IN source) SUBSTRING (source FROM n FOR len) TRANSLATE (string USING trans) TRIM (BOTH char FROM string) TRIM (LEADING char FROM string) TRIM (TRAILING char FROM string) UPPER (string) Returns The number of bits in a bit string The value, converted to the specified data type (e.g., a date converted to a character string) The length of a character string A string converted as specified by a named conversion function The current date The current time, with the specified precision The current date and time, with the specified precision The specified part (DAY, HOUR, etc.) from a DATETIME value A string converted to all lowercase letters The number of 8-bit bytes in a character string The position where the target string appears within the source string A portion of the source string, beginning at the nth character, for a length of len A string translated as specified by a named translation function A string with both leading and trailing occurrences of char trimmed off A string with any leading occurrences of char trimmed off A string with any trailing occurrences of char trimmed off A string converted to all uppercase letters
TABLE 5-6
SQL Standard Built-In Functions
Part II:
Retrieving Data
Missing Data (NULL Values)
Because a database is usually a model of a real-world situation, certain pieces of data are inevitably missing, unknown, or don t apply. In the sample database, for example, the QUOTA column in the SALESREPS table contains the sales goal for each salesperson. However, the newest salesperson has not yet been assigned a quota; this data is missing for that row of the table. You might be tempted to put a zero in the column for this salesperson, but that would not be an accurate reflection of the situation. The salesperson does not have a zero quota; the quota is just not yet known. Similarly, the MANAGER column in the SALESREPS table contains the employee number of each salesperson s manager. But Sam Clark, the vice president of sales, has no manager in the sales organization. This column does not apply to Sam. Again, you might think about entering a zero, or a 9999 in the column, but neither of these values would really be the employee number of Sam s boss. No data value is applicable to this row. SQL supports missing, unknown, or inapplicable data explicitly, through the concept of a null value. A null value is an indicator that tells SQL (and the user) that the data is missing or not applicable. As a convenience, a missing piece of data is often said to have the value NULL. But the NULL value is not a real data value like 0, 473.83, or Sam Clark. Instead, it s a signal, or a reminder, that the data value is missing or unknown. Figure 5-3 shows the contents of the SALESREPS table. Note that the QUOTA and REP_OFFICE values for Tom Snyder s row and the MANAGER value for Sam Clark s row of the table all contain NULL values. Also note that SQL tools do not display null values in query results in the same way while many use the string NULL as shown in Figure 5-3, others use empty space or character strings. In many situations, NULL values require special handling by the DBMS. For example, if the user requests the sum of the QUOTA column, how should the DBMS handle the missing data when computing the sum The answer is given by a set of special rules that govern NULL value handling in various SQL statements and clauses. Because of these rules, some leading database authorities feel strongly that NULL values should not be used. Others, including Ted Codd, have advocated the use of multiple NULL values, with distinct indicators for unknown and not applicable data.
FIGURE 5-3
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