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In the program shown in Figure 19-10, if one of the salesreps had a NULL value in his or her QUOTA column, the dbnextrow() call for that salesrep would retrieve a zero into the repquota variable. Note that the program cannot tell from the retrieved data whether the QUOTA column for the row really has a zero value, or whether it is NULL. In some applications, the use of substitution values is acceptable, but in others, it is important to be able to detect NULL values. These latter applications must use an alternative scheme for retrieving query results, described in the next section.
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With the basic SQL Server data retrieval technique, the dbnextrow() call copies the data value for each column into one of your program s variables. If there are many rows of query results or many long columns of text data, copying the data into your program s data areas can create significant overhead. In addition, the dbnextrow() call lacks a mechanism for returning NULL values to your program. To solve these two problems, dblib offers an alternate method of retrieving query results. Figure 19-13 shows the program excerpt from Figure 19-10, rewritten to use this alternate method: 1. The program sends the query to SQL Server and uses dbresults() to access the results, as it does for any SQL statement. However, the program does not call dbbind() to bind the columns of query results to program variables. 2. The program calls dbnextrow() to advance, row by row, through the query results. 3. For each column of each row, the program calls dbdata() to obtain a pointer to the data value for the column. The pointer points to a location within dblib s internal buffers. 4. If a column contains variable-length data, such as a VARCHAR data item, the program calls dbdatlen() to find the length of the data item. 5. If a column has a NULL value, the dbdata() function returns a null pointer (0), and dbdatlen() returns 0 as the length of the item. These return values give the program a way to detect and respond to NULL values in the query results. The program in Figure 19-13 is more cumbersome than the one in Figure 19-10. In general, it s easier to use the dbbind() function than the dbdata() approach, unless your program needs to handle NULL values or will be handling a large volume of query results.
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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
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FIGURE 19-13 Retrieval using the dbdata() function
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main() { LOGINREC *loginrec; /* data structure for login information */ char *namep; /* pointer to NAME column data */ int namelen; /* length of NAME column data */ float *quotap; /* pointer to QUOTA column data */ float *salesp; /* pointer to SALES column data */ char *namebuf; /* buffer to hold name */ /* Open a connection to SQL Server */ loginrec = dblogin(); DBSETLUSER(loginrec, "scott"); DBSETLPWD (loginrec, "tiger"); dbproc = dbopen(loginrec, ""); /* Pass query to dblib and ask SQL Server to execute it */ dbcmd(dbproc, "select name, quota, sales from salesreps "); dbcmd(dbproc, "where sales > quota order by name "); dbsqlexec(dbproc); /* Get to first statement in the batch */ dbresults(dbproc); /* Retrieve the single row of query results */ while (status = dbnextrow(dbproc) == SUCCEED) { /* Get the address of each data item in this row */ namep = dbdata(dbproc, 1); quotap = dbdata(dbproc, 2); salesp = dbdata(dbproc, 3); namelen = dbdatlen(dbproc, 1); /* Copy NAME value into our own buffer & null-terminate it */ strncpy(namebuf, namep, namelen); *(namebuf + namelen) = (char) 0; /* 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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