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The program in Figure 19-3 sends a single SQL statement to SQL Server and checks its status. If an application program must execute several SQL statements, it can repeat the dbcmd() / dbsqlexec() / dbresults() cycle for each statement. Alternatively, the program can send several statements as a single statement batch to be executed by SQL Server. Figure 19-4 shows a program that uses a batch of three SQL statements. As in Figure 19-3, the program calls dbcmd() to pass SQL text to dblib. The API simply
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main() { LOGINREC DBPROCESS /* Delete salespeople with low sales */ dbcmd(dbproc, delete from salesreps where sales < 10000.00"); /* Increase quota for salespeople with moderate sales */ dbcmd(dbproc, update salesreps set quota = quota + 10000.00"); dbcmd(dbproc, where sales <= 150000.00"); /* Increase quota for salespeople with high sales */ dbcmd(dbproc, update salesreps set quota = quota + 20000.00"); dbcmd(dbproc, where sales > 150000.00"); /* Ask SQL Server to execute the statement batch */ dbsqlexec(dbproc); /* if if if Check results of each (dbresults(dbproc) != (dbresults(dbproc) != (dbresults(dbproc) != of the three statements */ SUCCEED) goto do_error; SUCCEED goto do_error; SUCCEED goto do_error; *loginrec; *dbproc; /* data structure for login information */ /* data structure for connection */
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Figure 19-4.
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concatenates the text from each call. Note that it s the program s responsibility to include any required spaces or punctuation in the passed text. SQL Server does not begin executing the statements until the program calls dbsqlexec(). In this example, three statements have been sent to SQL Server, so the program calls dbresults() three times in succession. Each call to dbresults() advances the API to the results of the next statement in the batch and tells the program whether the statement succeeded or failed. In the program shown in Figure 19-4, the programmer knows in advance that three statements are in the batch, and the programmer can code three corresponding calls to dbresults(). If the number of statements in the batch is not known in advance, the program can call dbresults() repeatedly until it receives the error code NO_MORE_ RESULTS. The program excerpt in Figure 19-5 illustrates this technique.
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Error Handling
The value returned by the dbresults() function tells the program whether the corresponding statement in the statement batch succeeded or failed. To get more detailed information about a failure, your program must provide its own message-handling
/* Execute statements previously with dbcmd() calls */ dbsqlexec(dbproc); /* Loop checking results of each statement in the batch */ while (status = dbresults(dbproc) != NO_MORE_RESULTS { if (status == FAIL) goto handle_error; else printf( Statement succeeded.\n ); } /* Done with loop; batch completed successfully */ printf( Batch complete.\n ); exit();
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Figure 19-5.
Processing the results of a dblib statement batch
SQL: The Complete Reference
function. The dblib software automatically calls the message-handling function when SQL Server encounters an error while executing SQL statements. Note that dblib calls the message-handling function during its processing of the dbsqlexec() or dbresults() function calls, before it returns to your program (i.e., it is a callback function, called back by the SQL Server software). This allows the message-handling function to do its own error processing. Figure 19-6 shows an excerpt from a SQL Server program that includes a messagehandling function called msg_rtn(). When the program begins, it activates the message-handling function by calling msghandle(). Suppose an error occurs later, while SQL Server is processing the DELETE statement. When the program calls
/* External variables to hold error information */ int errcode; /* saved error code */ char errmsg[256]; /* saved error message */ /* Define our own message-handling function */ int msg_rtn(dbproc, msgno, msgstate, severity, msgtext) DBPROCESS *dbproc; DBINT msgno; int msgstate; int severity; char *msgtext; extern int errcode; extern char *errmsg; { /* Print out the error number and message */ printf( *** Error: %d Message: %s\n , msgno, msgtext); /* Save the error information for the application program */ errcode = msgno; strcpy(errmsg, msgtext); /* Return to dlib to complete the API call */ return(0); }
Figure 19-6.
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