barcode generator in vb.net 2005 ANSI/ISO SQL Data Types in Software

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TABLE 5-4 ANSI/ISO SQL Data Types
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Abbreviation(s) CHAR CHAR VARYING, VARCHAR CLOB NATIONAL CHAR, NCHAR NATIONAL CHAR VARYING, NCHAR NCLOB
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Description Fixed-length character strings Variable-length character strings Large fixed-length character strings Fixed-length national character strings Variable-length national character strings Large variable-length national character strings Fixed-length bit strings Variable-length bit strings
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Integers Small integers Decimal numbers
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Decimal numbers Floating point numbers Low-precision floating point numbers High-precision floating point numbers Calendar dates Clock times Clock times with time zones Dates and times Dates and times with time zones Time intervals Character data formatted as Extensible Markup Language (XML)
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SQL Basics
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Date/time data provides an excellent example of this phenomenon and the data type variations it creates. DB2 offered early date/time support, with three different date/time data types: DATE TIME Stores a date like June 30, 2008 Stores a time of day like 12:30:00 P.M.
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TIMESTAMP A specific instant in history, with a precision down to the nanosecond Specific dates and times can be specified as string constants, and date arithmetic is supported. Here is an example of a valid query using DB2 dates, assuming that the HIRE_ DATE column contains DATE data:
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SELECT NAME, HIRE_DATE FROM SALESREPS WHERE HIRE_DATE >= '05/30/2007' + 15 DAYS;
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PART II
SQL Server was introduced with a single date/time data type, called DATETIME, which closely resembles the DB2 TIMESTAMP data type. If HIRE_DATE contained DATETIME data, SQL Server could accept this version of the query (without the date arithmetic):
SELECT NAME, HIRE_DATE FROM SALESREPS WHERE HIRE_DATE >= '06/14/2007';
Since no specific time on June 14, 2007, is specified in the query, SQL Server defaults to midnight on that date. The SQL Server query thus really means
SELECT NAME, HIRE_DATE FROM SALESREPS WHERE HIRE_DATE >= '06/14/2007 12:00AM';
SQL Server also supports date arithmetic through a set of built-in functions. Thus, the DB2-style query can also be specified in this way:
SELECT NAME, HIRE_DATE FROM SALESREPS WHERE HIRE_DATE >= DATEADD(DAY, 15, '05/30/2007')
which is considerably different from the DB2 syntax. Oracle has long supported date/time data with a single data type called DATE. (Note, however, that Oracle added support for the SQL Standard DATETIME and TIMESTAMP data types starting with Oracle 9i.) Like SQL Server s DATETIME type, an Oracle DATE is, in fact, a timestamp. Also as with SQL Server, the time part of an Oracle DATE value defaults to midnight if no time is explicitly specified. The default Oracle date format is different from the DB2 and SQL Server formats, so the Oracle version of the query becomes
SELECT NAME, HIRE_DATE FROM SALESREPS WHERE HIRE_DATE >= '14-JUN-07';
Part II:
Retrieving Data
Oracle also supports limited date arithmetic, so the DB2-style query can also be specified, but without the DAYS keyword:
SELECT NAME, HIRE_DATE FROM SALESREPS WHERE HIRE_DATE >= '30-MAY-07' + 15;
Note, however, that this statement requires the DBMS to implicitly convert the string to an appropriate date data type before adding 15 to it, and that not all SQL implementations support such conversion. Oracle, for example, will report an error unless a function such as TO_DATE or CAST converts the character string to an Oracle DATE or DATETIME type before attempting date arithmetic. Fortunately, with the advent of the year 2000 conversion, most DBMS vendors added universal support for dates in SQL statements with four-digit years in a standard YYYYMM-DD format, which we use for most of the examples in this book. In Oracle s case, the default format is still as shown in the preceding examples, but it can be changed at either the database or user session with a simple command. If you are using Oracle and you try any of the examples in this book, simply enter this command to change your default date format:
ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD';
Care must be taken when forming queries that search for exact date matches using the equal (=) operator, and the dates have time components stored in them. Consider the following example:
SELECT NAME, HIRE_DATE FROM SALESREPS WHERE HIRE_DATE = '06/14/2007';
If a salesperson s hire date were stored in the database as noon on June 14, 2007, the salesperson would not be included in the query results from Oracle or SQL Server databases. The DBMS would assume a time of midnight for the string supplied with the SQL statement, and since midnight is not equal to noon, the row would not be selected. On the other hand, for a DB2 database, where the time is not stored with a DATE data type, the row would appear in the query results. Finally, starting with SQL2, the ANSI/ISO standard added support for date/time data with a set of data types based on, but not identical to, the DB2 types. In addition to the DATE, TIME, and TIMESTAMP data types, the standard specifies an INTERVAL data type, which can be used to store a time interval (for example, a timespan measured in days, or a duration measured in hours, minutes, and seconds). The standard also provides a very elaborate and complex method for dealing with date/time arithmetic, specifying the precision of intervals, adjusting for time zone differences, and so on. Most SQL implementations now have support for these standard types. One notable exception, however, is that SQL Server has long used the TIMESTAMP data type for an entirely different purpose, so supporting the ANSI/ISO specification for it presents a very real challenge. As these examples illustrate, the subtle differences in data types among various SQL products lead to some significant differences in SQL statement syntax.
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