free barcode generator source code in vb.net The Dynamic CLOSE Statement in Software

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The Dynamic CLOSE Statement
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The dynamic form of the CLOSE statement is identical in syntax and function to the static CLOSE statement shown in Figure 17-25. In both cases, the CLOSE statement ends
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Figure 18-15.
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The dynamic FETCH statement syntax diagram
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access to the query results. When a program closes a cursor for a dynamic query, the program normally should also deallocate the resources associated with the dynamic query, including: I The SQLDA allocated for the dynamic query and used in the DESCRIBE and FETCH statements I A possible second SQLDA, used to pass parameter values to the OPEN statement I The data areas allocated to receive each column of query results retrieved by a FETCH statement I The data areas allocated as indicator variables for the columns of query results It may not be necessary to deallocate these data areas if the program will terminate immediately after the CLOSE statement.
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Dynamic SQL Dialects
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Like the other parts of the SQL language, dynamic SQL varies from one brand of DBMS to another. In fact, the differences in dynamic SQL support are more serious than for static SQL, because dynamic SQL exposes more of the nuts and bolts of the underlying DBMS data types, data formats, and so on. As a practical matter, these differences make it impossible to write a single, general-purpose database front-end that is portable across different DBMS brands using dynamic SQL. Instead, database front-end programs must include a translation layer, often called a driver, for each brand of DBMS that they support to accommodate the differences. The early front-end products usually shipped with a separate driver for each of the popular DBMS brands. The introduction of ODBC as a uniform SQL API layer made this job simpler, since an ODBC driver could be written once for each DBMS brand, and the front-end program could be written to solely use the ODBC interface. In practice, however, ODBC s least-common-denominator approach meant that the front-end programs couldn t take advantage of the unique capabilities of the various supported DBMS systems, and it limited the performance of the application. As a result, most modern front-end programs and tools still include a separate, explicit driver for each of the popular DBMS brands. An ODBC driver is usually included to provide access to the others. A detailed description of the dynamic SQL features supported by all of the major DBMS brands is beyond the scope of this book. However, it is instructive to examine the dynamic SQL support provided by SQL/DS and by Oracle as examples of the kinds of differences and extensions to dynamic SQL that you may find in your particular DBMS.
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SQL: The Complete Reference
Dynamic SQL in Oracle *
The Oracle DBMS preceded DB2 into the market and based its dynamic SQL support on IBM s System/R prototype. For this reason, the Oracle support for dynamic SQL differs somewhat from the IBM SQL standard. Although Oracle and DB2 are broadly compatible, they differ substantially at the detail level. These differences include Oracle s use of parameter markers, its use of the SQLDA, the format of its SQLDA, and its support for data type conversion. The Oracle differences from DB2 are similar to those you may encounter in other DBMS brands. For that reason, it is instructive to briefly examine Oracle s dynamic SQL support and its points of difference from DB2.
Named Parameters
Recall that DB2 does not allow host variable references in a dynamically prepared statement. Instead, parameters in the statement are identified by question marks (parameter markers), and values for the parameters are specified in the EXECUTE or OPEN statement. Oracle allows you to specify parameters in a dynamically prepared statement using the syntax for host variables. For example, this sequence of embedded SQL statements is legal for Oracle:
exec sql begin declare section; char stmtbuf[1001]; int employee_number; exec sql end declare section; . . . strcpy(stmtbuf, "delete from salesreps where empl_num = :rep_number;"); exec sql prepare delstmt from :stmtbuf; exec sql execute delstmt using :employee_number;
Although rep_number appears to be a host variable in the dynamic DELETE statement, it is in fact a named parameter. As shown in the example, the named parameter behaves exactly like the parameter markers in DB2. A value for the parameter is supplied from a real host variable in the EXECUTE statement. Named parameters are a real convenience when you use dynamic statements with a variable number of parameters.
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