home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam    

3.3 Formatting Control Structures

The control structures in your program are the most direct representation of the logic needed to implement your specifications. The format of these control structures, therefore, will have a significant impact on the readability of your code.

Indentation is the most important element of control structure layout. Always keep statements of the same "logical level" at the same indentation level. Let's see what this means for the various control structures of PL/SQL.

3.3.1 Formatting IF Statements

This conditional construct comes in three flavors:

IF <expression>
END IF;
IF <expression>
ELSE
END IF;
IF <expression>
ELSEIF <expression>
ELSE
END IF;

In general, the IF statement is composed of clauses in which there is a Boolean expression or condition and a section of code executed when that condition evaluates to TRUE.

So if you want to use indentation to reveal the logical structure of the simplest form of the IF statement (IF-END IF), I suggest one of these two styles:

New Line for THEN Same Line for THEN
IF <expression>
THEN
   executable_statements;
END IF;
IF <expression> THEN
   executable_statements
END IF;
IF <expression>
THEN
   executable_statements;
ELSE
   else_executable_statements;
END IF;
IF <expression> THEN
   executable_statements
ELSE
   else_executable_statements;
END IF;
IF <expression>1
THEN
   executable_statements1;

ELSEIF <expression2>
THEN
   executable_statements2;
...

ELSEIF <expressionN>
THEN
   executable_statementsN;

ELSE
   else_executable_statements;
END IF;
IF <expression>1 THEN
   executable_statements1;

ELSEIF <expression2> THEN
   executable_statements2;
...

ELSEIF <expressionN> THEN
   executable_statementsN;

ELSE
   else_executable_statements;
END IF;

Notice that in both versions the executable statements are indented three spaces from the column in which the IF and END IF reserved words are found. The only difference between the two formats is the placement of the THEN reserved word. I prefer the new line format, in which the THEN appears on a line by itself after the IF condition. This format provides more whitespace than the other. I could create the whitespace by using a blank, rather than indenting three spaces, but then the executable statements for the IF clause are made distinct from the condition -- and they are logically connected. Let's examine some actual code to get a better sense of the differences.

The following example shows proper IF statement indentation with THEN on the same line:

IF max_sales > 2000 THEN
   notify_accounting ('over_limit');
   RAISE FORM_TRIGGER_FAILURE;
END IF;

This code has proper IF statement indentation with THEN on the next line:

IF max_sales > 2000
THEN
   notify_accounting ('over_limit');
   RAISE FORM_TRIGGER_FAILURE;
END IF;

3.3.2 Formatting Loops

You are going to be writing many loops in your PL/SQL programs, and they will usually surround some of the most complicated code in your application. For this reason, the format you use to structure your loops will make a critical difference in the overall comprehensibility of your programs.

PL/SQL offers the following kinds of loops:

  • Infinite or simple loop

  • WHILE loop

  • Indexed FOR loop (numeric and cursor)

Each loop has a loop boundary (begin and end statements) and a loop body. The loop body should be indented from the boundary (again, I recommend three spaces of indentation).

As with the IF statement, you can either choose to leave the LOOP reserved word at the end of the line containing the WHILE and FOR statements or place it on the next line. I prefer the latter, because then both the LOOP and END LOOP reserved words appear at the same column position (indentation) in the program.

Here are my recommendations for formatting your loops:

  • The infinite or simple loop:

LOOP
   executable_statements;
END LOOP;
  • The WHILE loop:

    WHILE condition
    LOOP
       executable_statements;
    END LOOP;
  • The numeric and cursor FOR loops:

    FOR for_index IN low_value .. high_value
    LOOP
       executable_statements;
    END LOOP;
    
    FOR record_index IN my_cursor
    LOOP
       executable_statements;
    END LOOP;

3.3.3 Formatting Exception Handlers

PL/SQL provides a very powerful facility for dealing with errors. An entirely separate exception section contains one or more "handlers" to trap exceptions and execute code when that exception occurs. Logically, the exception section is structured like a conditional CASE statement (which, by the way, is not supported by PL/SQL).

As you might expect, the format for the exception section should resemble that of an IF statement. Here is a general example of the exception section:

EXCEPTION
   WHEN NO_DATA_FOUND
   THEN
      executable_statements1;

   WHEN DUP_VAL_ON_INDEX
   THEN
      executable_statements1;

   ...
   WHEN OTHERS
   THEN
      otherwise_code;
END;

Instead of an IF or ELSIF keyword, the exception handler uses the word WHEN. In place of a condition (Boolean expression), the WHEN clause lists an exception name followed by a THEN and finally the executable statements for that exception. In place of ELSE, the exception section offers a WHEN OTHERS clause.

Follow these guidelines:

  • Indent each WHEN clause in from the EXCEPTION keyword that indicates the start of the exception section, as I've shown above. Place the THEN directly below the WHEN.

  • Indent all the executable statements for that handler in from the THEN keyword.

  • Place a blank line before each WHEN (except for the first).


Previous: 3.2 Formatting SQL Statements Oracle PL/SQL Programming, 2nd Edition Next: 3.4 Formatting PL/SQL Blocks
3.2 Formatting SQL Statements Book Index 3.4 Formatting PL/SQL Blocks

The Oracle Library Navigation

Copyright (c) 2000 O'Reilly & Associates. All rights reserved.

Library Home Oracle PL/SQL Programming, 2nd. Ed. Guide to Oracle 8i Features Oracle Built-in Packages Advanced PL/SQL Programming with Packages Oracle Web Applications Oracle PL/SQL Language Pocket Reference Oracle PL/SQL Built-ins Pocket Reference










??????????????@Mail.ru