The Proper Way to Say Goodbye
One important and fundamental principle in structured programming is "one way in, one way out." "One way in" is not an issue with PL/SQL: no matter what kind of loop you are using, there is always only one entry point into the loop—the first executable statement following the LOOP keyword. It is quite possible, however, to construct loops that have multiple exit paths. Avoid this practice. Having multiple ways of terminating a loop results in code that is much harder to debug and maintain.
In particular, you should follow these guidelines for loop termination:
· Do not use EXIT or EXIT WHEN statements within FOR and WHILE loops. You should use a FOR loop only when you want to iterate through all the values (integer or record) specified in the range. An EXIT inside a FOR loop disrupts this process and subverts the intent of that structure. A WHILE loop, on the other hand, specifies its termination condition in the WHILE statement itself.
· Do not use the RETURN or GOTO statements within a loop—again, these cause the premature, unstructured termination of the loop. It can be tempting to use these constructs because in the short run they appear to reduce the amount of time spent writing code. In the long run, however, you (or the person left to clean up your mess) will spend more time trying to understand, enhance, and fix your code over time.
Let's look at an example of loop termination issues with the cursor FOR loop. As you have seen, the cursor FOR loop offers many advantages when you want to loop through all of the records returned by a cursor. This type of loop is not appropriate, however, when you need to apply conditions to each fetched record to determine if you should halt execution of the loop. Suppose that you need to scan through each record from a cursor and stop when a total accumulation of a column (like the number of pets) exceeds a maximum, as shown in the following code. Although you can do this with a cursor FOR loop by issuing an EXIT statement inside the loop, that is an inappropriate use of this construct:
1 DECLARE 2 CURSOR occupancy_cur IS 3 SELECT pet_id, room_number 4 FROM occupancy WHERE occupied_dt = TRUNC (SYSDATE;) 5 pet_count INTEGER := 0; 6 BEGIN 7 FOR occupancy_rec IN occupancy_cur 8 LOOP 9 update_bill 10 (occupancy_rec.pet_id, occupancy_rec.room_number);11 pet_count := pet_count + 1;12 EXIT WHEN pet_count >= :GLOBAL.max_pets;13 END LOOP;14 END;The FOR loop explicitly states: "I am going to execute the body of this loop n times" (where n is a number in a numeric FOR loop, or the number of records in a cursor FOR loop). An EXIT inside the FOR loop (line 12) short-circuits this logic. The result is code that's difficult to follow and debug.
If you need to terminate a loop based on information fetched by the cursor FOR loop, you should use a WHILE loop or a simple loop in its place. Then the structure of the code will more clearly state your intentions.