Cursors

A cursor is a variable that runs through the tuples of some relation. This relation can be a stored table, or it can be the answer to some query. By fetching into the cursor each tuple of the relation, we can write a program to read and process the value of each such tuple. If the relation is stored, we can also update or delete the tuple at the current cursor position.
The example below illustrates a cursor loop. It uses our example relation T1(e,f) whose tuples are pairs of integers. The program will delete every tuple whose first component is less than the second, and insert the reverse tuple into T1.

1) DECLARE

/* Output variables to hold the result of the query: */

2) a T1.e%TYPE;

3) b T1.f%TYPE;

/* Cursor declaration: */

4) CURSOR T1Cursor IS

5) SELECT e, f

6) FROM T1

7) WHERE e < f

8) FOR UPDATE;

9) BEGIN

10) OPEN T1Cursor;

11) LOOP

/* Retrieve each row of the result of the above query

into PL/SQL variables: */

12) FETCH T1Cursor INTO a, b;

/* If there are no more rows to fetch, exit the loop: */

13) EXIT WHEN T1Cursor%NOTFOUND;

/* Delete the current tuple: */

14) DELETE FROM T1 WHERE CURRENT OF T1Cursor;

/* Insert the reverse tuple: */

15) INSERT INTO T1 VALUES(b, a);

16) END LOOP;

/* Free cursor used by the query. */

17) CLOSE T1Cursor;

18) END;

19) .

20) run;
Here are explanations for the various lines of this program:
Line (1) introduces the declaration section.
Lines (2) and (3) declare variables a and b to have types equal to the types of attributes e and f of the relation T1. Although we know these types are INTEGER, we wisely make sure that whatever types they may have