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  SQL Server Tips by Burleson

Handling different data types

xp_palindrome, srv_willconvert and srv_convert

This XP takes an input of any data type and returns true if the input is a palindrome. The returned value is stored in an output parameter of data type boolean. The XP works with character, numeric and binary data.

In TSQL all data conversions or castings both implicit and explicit, convert the value of the data from one data type to another based on the data representation and not the underlying raw data. For example an int is stored as four bytes and it can be converted to binary or CHAR:

DECLARE @i int
SET @i=12345
PRINT CAST(@i as varbinary)
PRINT CAST(@i as varchar)

Output:
0x00003039
12345


The variable @i is stored in four bytes and it can be converted to a binary data type with the exact same raw data. In contrast, converting to a varchar changes the length of the data from four to five bytes as well as modifying its contents. Instead of storing the value of @i in a 32-bit binary number, the data will be stored in five bytes, each one with one ASCII character from the value of @i.
Converting from int to real will have no loss of precision due to rounding, in this example. The two data types are stored in four bytes of data but the data is quite different, despite representing the same value:

PRINT CAST(@i as real)
PRINT CAST( CAST(@i as real) as varbinary)

Output:
12345
0x4640E400

Data type conversions by using C++ functions like atoi, atof, MultiByteToWideChar, etc. are possible in an XP but not necessary. ODS comes with several functions for simplifying the process of data type conversions. The two more interesting functions are srv_willconvert and srv_convert.

The srv_willconvert function returns TRUE if a conversion between two data types is legal and FALSE otherwise. For example, binary to real is not accepted. This function is very helpful because it will reject unacceptable conversions. It is almost always used preceding a call to srv_convert.

The srv_convert function will convert from one ODS data type to another. It mirrors the data type conversion functionality, which is available in TSQL.

int srv_convert (SRV_PROC * srvproc, int srctype, void * src, DBINT srclen, int desttype, void * dest, DBINT destlen )


The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter

 http://www.rampant-books.com/book_2005_2_sql_server_external_procedures.htm  

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