qr barcode generator vb.net The Standard SQLDA in Software

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The Standard SQLDA
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Although its support for PREPARE/EXECUTE processing closely parallels that of DB2 dynamic SQL, the SQL standard diverges substantially from DB2 style in the area of dynamic query processing. In particular, the SQL standard includes major changes to the DB2 SQL Data Area (SQLDA), which is at the heart of dynamic multirow queries. Recall that a SQL Data Area (SQLDA) provides two important functions: A flexible way to pass parameters to be used in the execution of a dynamic SQL statement (passing data from the host program to the DBMS), as described earlier in the section EXECUTE with SQLDA The way that the query results are returned to the program in the execution of a dynamic SQL query (passing data from the DBMS back to the host program), as described earlier in the section The Dynamic FETCH Statement The DB2-style SQLDA handles these functions with flexibility, but it has some serious disadvantages. It is a very low-level data structure, which tends to be specific to a particular programming language. For example, the variable-length structure of a DB2-style SQLDA makes it very difficult to represent in the FORTRAN language. The SQLDA structure also implicitly makes assumptions about the memory of the computer system on which the dynamic SQL program is running, how data items in a structure are aligned on such a system, and so on. For the writers of the SQL standard, these low-level dependencies were unacceptable barriers to portability. Therefore, they replaced the DB2 SQLDA structure with a set of statements for manipulating a more abstract structure called a dynamic SQL descriptor. The structure of a SQL descriptor is shown in Figure 18-18. Conceptually, the SQL descriptor is parallel to, and plays exactly the same role as, the DB2-style SQLDA shown in Figure 18-7. The fixed part of the SQL descriptor specifies a count of the number of items in the variable part of the descriptor. Each item in the variable part contains information about a single parameter being passed, such as its data type, its length, an indicator telling whether a NULL value is being passed, and so on. But unlike the DB2 SQLDA, the SQL descriptor is not an actual data structure within the host program. Instead, it is a collection of data items owned by the DBMS software. The host program manipulates SQL descriptors creating them, destroying them, placing data items into them, extracting data from them via a new set of dynamic SQL statements specially designed for that purpose. Figure 18-19 summarizes these SQL descriptor management statements.
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Dynamic SQL*
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FIGURE 18-18 SQL standard descriptor structure
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Fixed part
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COUNT
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number of items described
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Variable part one occurrence per item (parameter or query results column): data type of item TYPE LENGTH OCTET_LENGTH RETURNED_LENGTH RETURNED_OCTET_LENGTH PRECISION SCALE DATETIME_INTERVAL_CODE DATETIME_INTERVAL_PRECISION NULLABLE INDICATOR DATA NAME UNNAMED length of item length of item (in 8-bit octets) length of returned data item length of returned data (in 8-bit octets) precision of data item scale of data item type of date/time interval data precision of date/time interval data can item be NULL is data item NULL (indicator value) data item itself name of data item is data item unnamed
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To understand how the SQL descriptor management statements work, it s instructive to reexamine the dynamic SQL update program in Figure 18-8. This program illustrates the use of a DB2-style SQLDA in an EXECUTE statement. The flow of the program remains identical if a SQL descriptor is used instead, but the specifics change quite a lot.
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ALLOCATE DESCRIPTOR descriptor-name WITH MAX number-of-items
DEALLOCATE DESCRIPTOR descriptor-name
SET DESCRIPTOR COUNT = host variable VALUE item number item-name = host variable ,
PART V
GET DESCRIPTOR
host variable = COUNT VALUE item number host variable = item-name ,
FIGURE 18-19
SQL standard descriptor management statements
Part V:
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