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It s instructive to compare the programs in Figure 19-3 and Figure 19-4 to see the differences between the dblib approach and the embedded SQL: The embedded SQL program either implicitly connects to the only available database (as in DB2), or it includes an embedded SQL statement for connection (such as the CONNECT statement specified by the SQL standard). The dblib program connects to a particular SQL Server with the dbopen() call. The actual SQL UPDATE statement processed by the DBMS is identical in both programs. With embedded SQL, the statement is part of the program s source code. With dblib, the statement is passed to the API as a sequence of one or more character strings. In fact, the dblib approach more closely resembles the dynamic SQL EXECUTE IMMEDIATE statement than it does static SQL. In the embedded SQL program, host variables provide the link between the SQL statements and the values of program variables. With dblib, the program passes variable values to the DBMS in the same way that it passes program text as part of a SQL statement string. With embedded SQL, errors are returned in the SQLCODE or SQLSTATE field of the SQLCA structure. With dblib, the dbresults() call retrieves the status of each SQL statement. Overall, the embedded SQL program in Figure 19-4 is shorter and probably easier to read. However, the program is neither purely C nor purely SQL, and a programmer must be trained in the use of embedded SQL to understand it. The use of host variables means that the interactive and embedded forms of the SQL statement are different. In addition, the embedded SQL program must be processed both by the SQL precompiler and by the C compiler, lengthening the compilation cycle. In contrast, the SQL Server program is a plainvanilla C program, directly acceptable to the C compiler, and does not require special coding techniques.
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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 19-5 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 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 19-5, 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 19-6 illustrates this technique.
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PART V
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Part V:
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Programming with SQL
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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); /* Check results of each of the three statements */ if (dbresults(dbproc) != SUCCEED) goto do_error; if (dbresults(dbproc) != SUCCEED) goto do_error; if (dbresults(dbproc) != SUCCEED) goto do_error; . . . } *loginrec; *dbproc; /* data structure for login information */ /* data structure for connection */
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FIGURE 19-5
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