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FIGURE 19-14 Random row retrieval dblib
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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 re-retrieved 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 19-14. 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 SQL standard. In both 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.
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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 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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Later versions of SQL Server (and Sybase) added complete support for standard SQL cursors, with their associated DECLARE/OPEN/FETCH/CLOSE SQL statements. SQL Server and Sybase 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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Dynamic Queries
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In the program examples thus far in this chapter, the queries to be performed were known in advance. The columns of query results could be bound to program variables by explicit dbbind() calls hard-coded in the program. Most programs that use SQL Server can be written using this technique. (This static column binding corresponds to the fixed list of host variables used in the static SQL FETCH statement in standard embedded SQL, described in 17.) If the query to be carried out by a program is not known at the time the program is written, the program cannot include hard-coded dbbind() calls. Instead, the program must ask dblib for a description of each column of query results, using special API functions. The program can then bind the columns on the fly to data areas that it allocates at runtime. (This dynamic column binding corresponds to the use of the dynamic SQL DBNUMCOLS() statement and SQLDA, in dynamic embedded SQL, as described in 18.) Figure 19-15 shows an interactive query program that illustrates the dblib technique for handling dynamic queries. The program accepts a table name entered by the user and then prompts the user to choose which columns are to be retrieved from the table. As the user selects the columns, the program constructs a SELECT statement and then uses these steps to execute the SELECT statement and display the data from the selected columns: 1. The program passes the generated SELECT statement to SQL Server using the dbcmd() call, requests its execution with the dbsqlexec() call, and calls dbresults() to advance the API to the query results, as it does for all queries. 2. The program calls dbnumcols() to find out how many columns of query results were produced by the SELECT statement. 3. For each column, the program calls dbcolname() to find out the name of the column, and calls dbcoltype() to find out its data type. 4. The program allocates a buffer to receive each column of query results and calls dbbind() to bind each column to its buffer. 5. When all columns have been bound, the program calls dbnextrow() repeatedly to retrieve each row of query results.
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