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dbnextrow() includes a column with a NULL value, SQL Server replaces the NULL with a NULL substitution value. By default, SQL Server uses zero as a substitution value for numeric data types, a string of blanks for fixed-length strings, and an empty string for variable-length strings. The application program can change the default value for any data type by calling the API function dbsetnull(). In the program shown in Figure 19-7, if one of the offices had a NULL value in its QUOTA column, the dbnextrow() call for that office would retrieve a zero into the quota_value 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 a 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 19-8 shows the program excerpt from Figure 19-7, 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 out 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 19-8 is more cumbersome than the one in Figure 19-7. 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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main() { LOGINREC char int float float *loginrec; *namep; citylen; *quotap; *salesp; /* data structure for login information */ /* data structure for connection */ /* pointer to NAME column data */ /* length of NAME column data */ /* pointer to QUOTA column data */ /* pointer to SALES column data */ DBPROCESS *dbproc;
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/* 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;
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