5.1.1 Examples of Different Loops

To give you a feeling for the way the different loops solve their problems in different ways, consider the following three procedures. In each case, the procedure executes the same body of code inside a loop:

set_rank (ranking_level);

where set_rank performs a ranking for the specified level.

The simple loop

My procedure accepts a maximum ranking as an argument and then sets the rank until the level exceeds the maximum. Notice the IF statement used to guard against executing the loop when the maximum rank is negative. Notice also the EXIT WHEN statement used to terminate the loop:

PROCEDURE set_all_ranks (max_rank_in IN INTEGER)

IS

ranking_level NUMBER (3) := 1;

BEGIN

LOOP

EXIT WHEN ranking_level > max_rank_in;

set_rank (ranking_level);

ranking_level := ranking_level + 1;

END LOOP;

END set_all_ranks;

The FOR loop

In this case, I rank for the fixed range of values, from 1 to the maximum number:

PROCEDURE set_all_ranks (max_rank_in IN INTEGER)

IS

BEGIN

FOR ranking_level IN 1 .. max_rank_in

LOOP

set_rank (ranking_level);

END LOOP;

END set_all_ranks;

The WHILE loop

My procedure accepts a maximum ranking as an argument and then sets the rank until the level exceeds the maximum. Notice that the condition that terminates the loop comes on the same line as the WHILE keyword:

PROCEDURE set_all_ranks (max_rank_in IN INTEGER)

IS

ranking_level NUMBER(3) := 1;

BEGIN

WHILE ranking_level <= max_rank_in

LOOP

set_rank (ranking_level);

ranking_level := ranking_level + 1;

END LOOP;

END set_all_ranks;

In the above example, the FOR loop clearly requires the smallest amount of code. Yet I could use it in this case only because I knew that I would run the body of the loop a specific number of times (max_rank_in). In many other situations, the number of times a loop must execute varies, so the FOR loop cannot be used.

5.1.2 Structure of pl/sql Loops

While there are differences among the three loop constructs, every loop has two parts: the loop boundary and the loop body.

Loop boundary

This is composed of the reserved words that initiate the loop, the condition that causes the loop to terminate, and the END LOOP statement that ends the loop.

Loop body

This is the sequence of executable statements inside the loop boundary that execute on each iteration of the loop.

Figure 5-1 shows the boundary and body of aWHILE loop.

Figure 5-1. The boundary and body of the while loop

In general, think of a loop much as you would a procedure or a function. The body of the loop is a black box, and the condition that causes loop termination is the interface to that black box. Code outside the loop should not have to know about the inner workings of the loop. Keep this in mind as you go through the different kinds of loops and examples in the rest of the chapter.

In addition to the examples you will find in this chapter, I have included several lengthy code samples utilizing PL/SQL loops in the Oracle Forms environment in the following files, available on the O'Reilly site:

highrec.doc and highrec.fp

Demonstrate highlighting items in an Oracle Forms record.

ofquery.doc, postqry.fp, and preqry.fp

Demonstrate automatic post- and pre-query processing in Oracle Forms.