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The dynamic OPEN statement syntax diagram
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. . . /* Program has previously generated and prepared a SELECT statement like this one: SELECT A, B, C ... FROM SALESREPS WHERE SALES BETWEEN AND with two parameters to be specified */
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/* Prompt the user for low & high values and do the query */ printf("Enter low end of sales range: "); scanf("%f", &low_end); printf("Enter high end of sales range: "); scanf("%f", &high_end); /* Open the cursor to start the query, passing parameters */ exec sql open qrycursor using :low_end, :high_end; . . . .. . . .
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was described for the EXECUTE statement earlier in the section EXECUTE with SQLDA. The same technique is used for the OPEN statement. Figure 18-14 shows a program excerpt like the one in Figure 18-13, except that it uses a SQLDA to pass parameters. Note carefully that the SQLDA used in the OPEN statement has absolutely nothing to do with the SQLDA used in the DESCRIBE and FETCH statements: I The SQLDA in the OPEN statement is used to pass parameter values to the DBMS for dynamic query execution. The elements of its SQLVAR array correspond to the parameter markers in the dynamic statement text. I The SQLDA in the DESCRIBE and FETCH statements receives descriptions of the query results columns from the DBMS and tells the DBMS where to place the retrieved query results. The elements of its SQLVAR array correspond to the columns of query results produced by the dynamic query.
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. . . /* Program has previously generated and prepared a SELECT statement like this one: SELECT A, B, C ... FROM SALESREPS WHERE EMPL_NUM IN ( , , ... ) with a variable number of parameters to be specified. The number of parameters for this execution is stored in the variable parmcnt. */ char *malloc() SQLDA *parmda; SQLVAR *parmvar; long parm_value[101]; /* Allocate a SQLDA to pass parameter values */ parmda = (SQLDA *)malloc(sizeof(SQLDA) + parmcnt * sizeof(SQLVAR)); parmda->sqln = parmcnt; /*Prompt the user for parameter values */ for (i = 0; i < parmcnt; i++) { printf("Enter employee number: "); scanf("%ld", &(parm_value[i])); parmvar = parmda -> sqlvar + i; parmvar->sqltype = 496; parmvar->sqllen = 4; parmvar->sqldata = &(parm_value[i]); parmvar->sqlind = 0; } /* Open the cursor to start the query, passing parameters */ exec sql open qrycursor using descriptor :parmda; . . .
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Figure 18-14.
OPEN statement with SQLDA parameter passing
SQL: The Complete Reference
The Dynamic FETCH Statement
The dynamic FETCH statement, shown in Figure 18-15, is a variation of the static FETCH statement. It advances the cursor to the next available row of query results and retrieves the values of its columns into the program s data areas. Recall from 17 that the static FETCH statement includes an INTO clause with a list of host variables that receive the retrieved column values. In the dynamic FETCH statement, the list of host variables is replaced by a SQLDA. Before using the dynamic FETCH statement, it is the application program s responsibility to provide data areas to receive the retrieved data and indicator variable for each column. The application program must also fill in the SQLDATA, SQLIND, and SQLLEN fields in the SQLVAR structure for each column, as follows: I The SQLDATA field must point to the data area for the retrieved data. I The SQLLEN field must specify the length of the data area pointed to by the SQLDATA field. This value must be correctly specified to make sure the DBMS does not copy retrieved data beyond the end of the data area. I The SQLIND field must point to an indicator variable for the column (a 2-byte integer). If no indicator variable is used for a particular column, the SQLIND field for the corresponding SQLVAR structure should be set to zero. Normally, the application program allocates a SQLDA, uses the DESCRIBE statement to get a description of the query results, allocates storage for each column of query results, and sets the SQLDATA and SQLIND values, all before opening the cursor. This same SQLDA is then passed to the FETCH statement. However, there is no requirement that the same SQLDA be used or that the SQLDA specify the same data areas for each FETCH statement. It is perfectly acceptable for the application program to change the SQLDATA and SQLIND pointers between FETCH statements, retrieving two successive rows into different locations.
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