barcode generator vb.net free The Variant Datatype in Software

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The Variant Datatype
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The sql_variant datatype is based on the same idea as the variant datatype in Visual Basic. It is designed to allow a single variable, column, or parameter to store values in different datatypes. Internally, variant objects record two values:
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The actual value The metadata describing the variant: base datatype, maximum size, scale, precision, and collation
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Basic Transact-SQL Programming Constructs
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The following statement creates a lookup table that can store values of different types:
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Create table Lookup( LookupGroupId tinyint, LookupId smallint, LookupValue sql_variant) Go
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Before SQL Server 2000, I had to use more than one field to store lookup values of different datatypes. The following statements illustrate how you can insert different types of values in one column:
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Insert Lookup (LookupGroupId, LookupId, LookupValue) Values (2, 34, 'VAR') Insert Lookup (LookupGroupId, LookupId, LookupValue) Values (3, 22, 2000) Insert Lookup (LookupGroupId, LookupId, LookupValue) Values (4, 16, '1/12/2000') Insert Lookup (LookupGroupId, LookupId, LookupValue) Values (4, 11, $50000)
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A sql_variant object can store values of any datatype except:
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But there are more serious restrictions on their use:
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sql_variant columns cannot be part of primary or foreign keys. sql_variant columns can be used in indexes and unique keys as long as their values are shorter than 900 bytes. sql_variant columns cannot have an identity property.
SQL Server 2000 Stored Procedure Programming
sql_variant columns cannot be part of a computed column. Developers must use functions for converting datatypes when assigning values from sql_variant objects to objects of other datatypes. The comparison of sql_variant objects has complex rules and is prone to errors. sql_variant values are automatically converted to nvarchar(4000)when accessed from client applications using OLE DB Provider for SQL Server 7.0 or the SQL Server ODBC Driver from SQL Server version 7.0. If stored values are longer then 4,000 characters, SQL Server will return just the first 4,000 characters. sql_variant values are automatically converted to varchar(255) when accessed from client applications using the SQL Server ODBC Driver from SQL Server version 6.5 or earlier, or using DB-Library. If stored values are longer then 255 characters, SQL Server will return just the first 255 characters. sql_variant columns are not supported in the Like predicate. sql_variant columns do not support full-text indexes. sql_variant objects cannot be concatenated using the + operator, even if the stored values are strings or numeric. The proper solution is to convert values before concatenation. Some functions (AVG, IDENTITY, ISNUMERIC, POWER, RADIANS, ROUND, SIGN, STDEV[P], SUM, VAR[P]) do not support sql_variant parameters.
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TIP: You should be very conservative in using the sql_variant datatype. Its use has serious performance and design implications.
4:
Basic Transact-SQL Programming Constructs
The Cursor Datatype
This is a special kind of datatype that can contain references to cursors. You will see later in this chapter that cursors are programming constructs that are designed to allow operations on records one at a time. It is not possible to define a column of this type. It can be used only for variables and stored procedure output values.
The Table Datatype
The table datatype is used to store a recordset for later processing. In some ways, this datatype is similar to a temporary table. It was introduced in SQL Server 2000. You cannot use this type to define a column. It can only be used as a local variable to return the value of a function.
NOTE: You will find more information about table variables later in this chapter, and information about functions in s 5 and 9.
Datatype Synonyms
In some cases, developers can use different identifiers to refer to a datatype. For example, the char datatype can be referenced as character and varchar as character varying. Some of these synonyms are based on ANSI SQL-92 standard requirements.
NOTE: Appendix A contains a table with a list of datatypes and their descriptions, ranges, sizes, and sample constants.
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