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The positioned UPDATE statement syntax diagram
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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] /*
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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 */
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/* 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 = cust_num 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);
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Embedded SQL
I The query cannot specify an ORDER BY clause; the cursor must not identify a sorted set of query results. I The query cannot specify the DISTINCT keyword. I The query must not include a GROUP BY or a HAVING clause. I The user must have the UPDATE or DELETE privilege (as appropriate) on the base table. The IBM databases (DB2, 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 language. Because of its widespread use, the SQL2 standard includes the IBM-style FOR UPDATE clause as an option in its DECLARE CURSOR statement. However, unlike the IBM products, the SQL2 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 SQL2 DECLARE CURSOR statement appears in exactly the same position as the FOR UPDATE clause and explicitly tells the DBMS that the program will not attempt a positioned DELETE or UPDATE operation using the cursor. Because they can significantly affect database overhead and performance, it can be very
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Figure 17-33.
The DECLARE CURSOR statement with FOR UPDATE clause
SQL: The Complete Reference
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.
Cursors and Transaction Processing
The way your program handles its cursors can have a major impact on database performance. Recall from 12 that the SQL transaction model guarantees the consistency of data during a transaction. In cursor terms, this means that your program can declare a cursor, open it, fetch the query results, close it, reopen it, and fetch the query results again and be guaranteed that the query results will be identical both times. The program can also fetch the same row through two different cursors and be guaranteed that the results will be identical. In fact, the data is guaranteed to remain consistent until your program issues a COMMIT or ROLLBACK to end the transaction. Because the consistency is not guaranteed across transactions, both the COMMIT and ROLLBACK statements automatically close all open cursors. Behind the scenes, the DBMS provides this consistency guarantee by locking all of the rows of query results, preventing other users from modifying them. If the query produces many rows of data, a major portion of a table may be locked by the cursor. Furthermore, if your program waits for user input after fetching each row (for example, to let the user verify data displayed on the screen), parts of the database may be locked for a very long time. In an extreme case, the user might leave for lunch in mid-transaction, locking out other users for an hour or more! To minimize the amount of locking required, you should follow these guidelines when writing interactive query programs: I Keep transactions as short as possible. I Issue a COMMIT statement immediately after every query and as soon as possible after your program has completed an update. I Avoid programs that require a great deal of user interaction or that browse through many rows of data. I If you know that the program will not try to refetch a row of data after the cursor has moved past it, use one of the less restrictive isolation modes described in 12. This allows the DBMS to unlock a row as soon as the next FETCH statement is issued. I Avoid the use of scroll cursors unless you have taken other actions to eliminate or minimize the extra database locking they will cause. I Explicitly specify a READ ONLY cursor, if possible.
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