Using FOR UPDATE
If you declare a cursor which will be referenced in the CURRENT OF clause of an UPDATE or DELETE statement, you should use the FOR UPDATE clause to obtain an exclusive row locks. An illustration is as shown below:
DECLARE
CURSOR c1 IS SELECT empno, sal FROM emp
WHERE job = 'SALESMAN' AND comm > sal
FOR UPDATE NOWAIT;
The FOR UPDATE clause identifies the row which will be updated or deleted, then locks each & every row in the result set. This is helpful when you want to base an update on the existing values in a row. In that situation, you should make sure that the row is not changed by the other user before the update.
The elective keyword NOWAIT tells the Oracle not to wait if the table has been locked by the other user. The Control is immediately returned to your program so that it can do the other work before trying again to obtain the lock. If you omit the keyword NOWAIT, the Oracle waits until the table is available.
All rows are locked when you open the cursor, they are not liked fetched. The rows are unlocked when you commit or roll back the transaction. And hence, you cannot fetch from a
When querying the multiple tables, you can use the FOR UPDATE clause to lock up the row locking to the particular tables. The Rows in a table are locked only if the FOR UPDATE OF the clause refers to the column in that table. For illustration, the following query locks rows in the emp table but not in the dept table:
DECLARE
CURSOR c1 IS SELECT ename, dname FROM emp, dept
WHERE emp.deptno = dept.deptno AND job = 'MANAGER'
FOR UPDATE OF sal;
As the next illustration shows, you use the CURRENT OF clause in an UPDATE or DELETE statement to refer to the newest row fetched from a cursor:
DECLARE
CURSOR c1 IS SELECT empno, job, sal FROM emp FOR UPDATE;
...
BEGIN
OPEN c1;
LOOP
FETCH c1 INTO ...
...
UPDATE emp SET sal = new_sal WHERE CURRENT OF c1;
END LOOP;