qr barcode generator vb.net Cursor-Based Deletes and Updates in Software

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Cursor-Based Deletes and Updates
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Application programs often use cursors to allow the user to browse through a table of data row by row. For example, the user may ask to see all of the orders placed by a particular customer. The program declares a cursor for a query of the ORDERS table and displays each
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FIGURE 17-30
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The positioned DELETE statement syntax diagram
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order on the screen, possibly in a computer-generated form, waiting for a signal from the user to advance to the next row. Browsing continues in this fashion until the user reaches the end of the query results. The cursor serves as a pointer to the current row of query results. If the query draws its data from a single table and it is not a summary query, as in this example, the cursor implicitly points to a row of a database table, because each row of query results is drawn from a single row of the table. While browsing the data, the user may spot data that should be changed. For example, the order quantity in one of the orders may be incorrect, or the customer may want to delete one of the orders. In this situation, the user wants to update or delete this order. The row is not identified by the usual SQL search condition; rather, the program uses the cursor as a pointer to indicate which row is to be updated or deleted. Embedded SQL supports this capability through special versions of the DELETE and UPDATE statements, called the positioned DELETE and positioned UPDATE statements, respectively. The positioned DELETE statement, shown in Figure 17-30, deletes a single row from a table. The deleted row is the current row of a cursor that references the table. To process the statement, the DBMS locates the row of the base table that corresponds to the current row of the cursor and deletes that row from the base table. After the row is deleted, the cursor has no current row. Instead, the cursor is effectively positioned in the empty space left by the deleted row, waiting to be advanced to the next row by a subsequent FETCH statement. The positioned UPDATE statement, shown in Figure 17-31, updates a single row of a table. The updated row is the current row of a cursor that references the table. To process the statement, the DBMS locates the row of the base table that corresponds to the current row of the cursor and updates that row as specified in the SET clause. After the row is updated, it remains the current row of the cursor. Figure 17-32 shows an order-browsing program that uses the positioned UPDATE and DELETE statements: 1. The program first prompts the user for a customer number and then queries the ORDERS table to locate all of the orders placed by that customer. 2. As it retrieves each row of query results, it displays the order information on the screen and asks the user what to do next. 3. If the user types an N, the program does not modify the current order, but moves directly to the next order.
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FIGURE 17-31 The positioned UPDATE statement syntax diagram
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Programming with SQL
main() { exec sql include sqlca; exec sql begin declare section; int custnum; int ordnum; char orddate[12]; char ordmfr[4]; char ordproduct[6]; int ordqty; float ordamount; exec sql end declare section; char inbuf[101]
/* /* /* /* /* /* /*
customer number entered by user */ retrieved order number */ retrieved order date */ retrieved manufacturer-id */ retrieved product id */ retrieved order quantity */ retrieved order amount */
/* character entered by user */
/* Declare the cursor for the query */ exec sql declare ordcurs cursor for select order_num, ord_date, mfr, product, qty, amount from orders where cust = custnum order by order_num for update of qty, amount; /* Prompt the user for a customer number */ printf("Enter customer number:"); scanf("%d", &custnum); /* Set up error processing */ whenever sqlerror goto error; whenever not found goto done; /* Open the cursor to start the query */ exec sql open ordcurs; /* Loop through each row of query results */ for (;;) { /* Fetch the next row of query results */ exec sql fetch ordcurs into :ordnum, :orddate, :ordmfr, :ordproduct, :ordqty, :ordamount; /* Display the retrieved data */ printf("Order Number: %d\n", ordnum); printf("Order Date: %s\n", orddate); printf("Manufacturer: %s\n", ordmfr); printf("Product: %s\n", ordproduct); printf("Quantity: %s\n", ordqty); printf("Total Amount: %f\n", ordamount); /* Prompt user for action on this order */ printf("Enter action (Next/Delete/Update/Exit): "); gets(inbuf); 1
FIGURE 17-32
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