qr barcode generator vb.net Using the positioned DELETE and UPDATE statements in Software

Generate QR Code ISO/IEC18004 in Software Using the positioned DELETE and UPDATE statements

Using the positioned DELETE and UPDATE statements
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switch (inbuf[0]) { case 'N': /* Continue on to the next order */ break; case 'D': /* Delete the current order */ exec sql delete from orders where current of ordcurs; break; case 'U': /* Update the current order */ printf("Enter new quantity: "); scanf("%d", &ordqty); printf("Enter new amount: "); scanf("%f", &ordamount); exec sql update orders set qty = :ordqty, amount = :ordamount where current of ordcurs; break; case 'X': /* Stop retrieved orders and exit */ goto done; } } done: exec sql close ordcurs; exec sql commit; exit(); error: printf("SQL error: %ld\n", sqlca.sqlcode); exit(); } 3
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Using the positioned DELETE and UPDATE statements (continued)
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4. If the user types a D, the program deletes the current order using a positioned DELETE statement. 5. If the user types a U, the program prompts the user for a new quantity and amount, and then updates these two columns of the current order using a positioned UPDATE statement. 6. If the user types an X, the program halts the query and terminates.
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Although it is primitive compared with a real application program, the example in Figure 17-32 shows all of the logic and embedded SQL statements required to implement a browsing application with cursor-based database updates. The SQL1 standard specified that the positioned DELETE and UPDATE statements can be used only with cursors that meet these very strict criteria: The query associated with the cursor must draw its data from a single source table; that is, there must be only one table named in the FROM clause of the query specified in the DECLARE CURSOR statement. The query cannot specify an ORDER BY clause; the cursor must not identify a sorted set of query results. The query cannot specify the DISTINCT keyword. The query must not include a GROUP BY or a HAVING clause. The user must have the UPDATE or DELETE privilege (as appropriate) on the base table. The IBM databases (DB2 and SQL/DS) extended the SQL1 restrictions a step further. They require that the cursor be explicitly declared as an updateable cursor in the DECLARE CURSOR statement. The extended IBM form of the DECLARE CURSOR statement is shown in Figure 17-33. In addition to declaring an updateable cursor, the FOR UPDATE clause can optionally specify particular columns that may be updated through the cursor. If the column list is specified in the cursor declarations, positioned UPDATE statements for the cursor may update only those columns. In practice, all commercial SQL implementations that support positioned DELETE and UPDATE statements follow the IBM SQL approach. It is a great advantage for the DBMS to know, in advance, whether a cursor will be used for updates or whether its data will be read-only, because read-only processing is simpler. The FOR UPDATE clause provides this advance notice and can be considered a de facto standard of the embedded SQL. Because of its widespread use, subsequent versions of the SQL standard include the IBM-style FOR UPDATE clause as an option in its DECLARE CURSOR statement. However, unlike the IBM products, the SQL standard automatically assumes that a cursor is opened for update unless it is a scroll cursor or it is explicitly declared FOR READ ONLY. The FOR READ ONLY specification in the DECLARE CURSOR statement appears in exactly the same position as the FOR UPDATE clause and explicitly tells the DBMS that the program will not
FIGURE 17-33
The DECLARE CURSOR statement with FOR UPDATE clause
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attempt a positioned DELETE or UPDATE operation using the cursor. Because updateable cursors can significantly affect database overhead and performance, it is very important to understand the specific assumptions that your particular DBMS brand makes about the updateability of cursors, and the clauses or statements that can be used to override them. In addition, programs that explicitly declare whether their intention is to allow updates via an opened cursor are more maintainable.
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