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Retrieval using the dbdata() function
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/* Print data for this salesperson */ printf( Name: %s\n , namebuf); if (quotap == 0) printf( Quota is NULL.\n ); else printf( Quota: %f\n , *quotap); printf( Sales: %f\n , *salesp); } /* Check for successful completion */ if (status == FAIL) printf( SQL error.\n ); dbexit(dbproc); exit(); }
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A program normally processes SQL Server query results by moving through them sequentially using the dbnextrow() call. For browsing applications, dblib also provides limited random access to the rows of query results. Your program must explicitly enable random row access by turning on a dblib option. The dbgetrow() call can then be used to retrieve a row by its row number. To support random row retrieval, dblib stores the rows of query results in an internal buffer. If the query results fit entirely within the dblib buffer, dbgetrow() supports random retrieval of any row. If the query results exceed the size of the buffer, only the initial rows of query results are stored. The program can randomly retrieve these rows, but a dbnextrow() call that attempts to retrieve a row past the end of the buffer returns the special BUF_FULL error condition. The program must then discard some of the saved rows from the buffer, using the dbclrbuf() call, to make room for the new row. Once the rows are discarded, they cannot be reretrieved with the dbgetrow() function. Thus, dblib supports random retrieval of query results within a limited window, dictated by the size of the row buffer, as shown in Figure 19-9. Your program can specify the size of the dblib row buffer by calling the dblib routine dbsetopt(). The random access provided by dbgetrow() is similar to the scroll cursors supported by several DBMS products and specified by the SQL2 standard. In both
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Figure 19-9.
Random row retrieval with dblib
cases, random retrieval by row number is supported. However, a scroll cursor is a true pointer into the entire set of query results; it can range from the first to the last row, even if the query results contain thousands of rows. By contrast, the dbgetrow() function provides random access only within a limited window. This is adequate for limited browsing applications but cannot easily be extended to large queries.
Positioned Updates
In an embedded SQL program, a cursor provides a direct, intimate link between the program and the DBMS query processing. The program communicates with the DBMS row by row as it uses the FETCH statement to retrieve query results. If the query is a simple single-table query, the DBMS can maintain a direct correspondence between the current row of query results and the corresponding row within the database. Using this correspondence, the program can use the positioned update statements (UPDATE WHERE CURRENT OF and DELETE WHERE CURRENT OF) to modify or delete the current row of query results. SQL Server query processing uses a much more detached, asynchronous connection between the program and the DBMS. In response to a statement batch containing one or more SELECT statements, SQL Server sends the query results back to the dblib software, which manages them. Row-by-row retrieval is handled by the dblib API calls, not by SQL language statements. As a result, early versions of SQL Server could not support positioned updates because its notion of a current row applied to query results within the dblib API, not to rows of the actual database tables.
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SQL APIs
Later versions of SQL Server (and Sybase) added complete support for standard SQL cursors, with their associated DECLARE/OPEN/FETCH/CLOSE SQL statements. Cursors actually operate within Transact-SQL stored procedures, and the action of the FETCH statement is to fetch data from the database into the stored procedure for processing not to actually retrieve it into the application program that called the stored procedure. Stored procedures and their operation within various popular SQL DBMS products are discussed in 20.
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