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PROGRAMMING WITH SQL
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SQL: The Complete Reference
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The programming required to perform a dynamic query is more extensive than the programming for any other embedded SQL statement. However, the programming is typically more tedious than complex. Figure 18-9 shows a small query program that uses dynamic SQL to retrieve and display selected columns from a user-specified table. The callouts in the figure identify the eight steps in the preceding list.
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main() { /* This is a simple general-purpose query program. It prompts the user for a table name, and then asks the user which columns of the table are to be included in the query. After the user s selections are complete, the program runs the requested query and displays the results. */ exec sql include sqlca; exec sql include sqlda; exec sql begin declare section; char stmtbuf[2001]; char querytbl[32]; char querycol[32]; exec sql end declare section;
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/* SQL text to be executed */ /* user-specified table */ /* user-specified column */
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/* Cursor for system catalog query that retrieves column names */ exec sql declare tblcurs cursor for select colname from system.syscolumns where tblname = :querytbl and owner = user; exec sql declare qrycurs cursor for querystmt; /* Data structures for the program int colcount = 0; struct sqlda *qry_da; struct sqlvar *qry_var; int i; */ char inbuf[101]; /* input entered by user */ */ /* /* /* /* 1
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number of columns chosen */ allocated SQLDA for query */ SQLVAR for current column */ index for SQLVAR array in SQLDA
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/* Prompt the user for which table to query */ printf("*** Mini-Query Program ***\n\n") printf("Enter name of table for query: "); gets(querytbl); /* Start the SELECT statement in the buffer */
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Data retrieval with dynamic SQL
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18:
Dynamic SQL *
strcpy(stmtbuf, "select "); /* Set up error processing */ exec sql whenever sqlerror goto handle_error; exec sql whenever not found goto no_more_columns;
/* Query the system catalog to get column names for the table */ exec sql open tblcurs; for ( ; ; ) { /* Get name of next column and prompt the user */ exec sql feth tblcurs into :querycol; printf("Include column %s (y/n) ", querycol); gets(inbuf); if (inbuf[0] == 'y') { /* User wants the column; add it to the select list */ if (colcount++ > 0) strcat(stmtbuf, ", "); strcat(stmtbuf, querycol); } } no_more_columns: exec sql close tblcurs; /* Finish the SELECT statement with a FROM clause */ strcat(stmtbuf, "from "); strcat(stmtbuf, querytbl); /* Allocate SQLDA for the dynamic query */ query_da = (SQLDA *)malloc(sizeof(SQLDA) + colcount * sizeof(SQLVAR)); query_da->sqln = colcount; /* Prepare the query and ask the DBMS to describe it */ exec sql prepare querystmt from :stmtbuf; exec sql describe querystmt into qry_da; /* Loop through SQLVARs, allocating memory for each column */ for (i = 0; i < colcount; i++) { qry_var = qry_da->sqlvar + i; qry_var->sqldat = malloc(qry_var->sqllen); qry_var->sqlind = malloc(sizeof(short)); } /* SQLDA is all set; do the query and retrieve the results! */
PROGRAMMING WITH SQL
Figure 18-9.
Data retrieval with dynamic SQL (continued)
SQL: The Complete Reference
exec sql open qrycurs; exec sql whenever not found goto no_more_data; for ( ; ; ) { /* Fetch the row of data into our buffers */ exec sql fetch sqlcurs using descriptor qry_da; printf("\n"); /* Loop printing data for each column of the row */ for (i = 0; i < colcount; i++) { /* Find the SQLVAR for this column; print column label */ qry_var = qry_da->sqlvar + i; printf(" Column # %d (%s): ", i+1, qry_var->sqlname); /* Check indicator variable for NULL indication */ if (*(qry_var -> sqlind)) != 0) { puts("is NULL!\n"); continue; } /* Actual data returned; handle each type separately */ switch (qry_var -> sqltype) { case 448: case 449: /* VARCHAR data - just display it */ puts(qry_var -> sqldata); break; case 496: case 497: /* Four-byte integer data - convert & display it */ printf("%ld", *((int *) (qry_var->sqldata))); break; case 500: case 501: /* Two-byte integer data - convert & dispay it */ printf("%d", *((short *)(qry_var->sqldata))); break; case 480: case 481: /* Floating-point data - convert & display it */
Figure 18-9.
Data retrieval with dynamic SQL (continued)
18:
Dynamic SQL *
printf("%lf", *((double *)(qry_var->sqldat))); break; } } } no_more_data: printf("\nEnd of data.\n"); /* Clean up allocated storage */ for (i = 0; i < colcount; i++) { qry_var = qry_da->sqlvar + i; free(qry_var->sqldata); free(qry_var->sqlind); } free(qry_da); close qrycurs; exit(); }
Figure 18-9.
Data retrieval with dynamic SQL (continued)
The program in the figure begins by prompting the user for the table name and then queries the system catalog to discover the names of the columns in that table. It asks the user to select the column(s) to be retrieved and constructs a dynamic SELECT statement based on the user s responses. The step-by-step mechanical construction of a select list in this example is very typical of database front-end programs that generate dynamic SQL. In real applications, the generated select list might include expressions or aggregate functions, and there might be additional program logic to generate GROUP BY, HAVING, and ORDER BY clauses. A graphical user interface would also be used instead of the primitive user prompting in the sample program. However, the programming steps and concepts remain the same. Notice that the generated SELECT statement is identical to the interactive SELECT statement that you would use to perform the requested query. The handling of the PREPARE and DESCRIBE statements and the method of allocating storage for the retrieved data in this program are also typical of dynamic query programs. Note how the program uses the column descriptions placed in the SQLVAR array to allocate a data storage block of the proper size for each column. This program also allocates space for an indicator variable for each column. The program places the address of the data block and indicator variable back into the SQLVAR structure.
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