PLSQL CASE

One can see CASE as a compact variation of IF THEN ELSE construct. Compact in a sense that you don't have to repeatedly encode the expression IF <condition> - however CASE is only applicable to scenarios wherein you need to test a variable against a set of possible values. 

CASE version

CASE stoplight
  WHEN 'red' THEN Action('Stop');
  WHEN 'yellow' THEN Action('SlowDown');
  WHEN 'green' THEN Action('Go');
  ELSE Action('Caution');
END CASE;

IF THEN ELSE version

IF stoplight = 'red' THEN 
  Action('Stop'); 
ELSIF stoplight = 'yellow' THEN
  Action('SlowDown'); 
ELSIF stoplight = 'green' THEN
  Action('Go'); 
ELSE
  Action('Caution'); 
END IF;

Some says that using CASE construct expresses the programmer's intent better and explicitly expresses that the given cases are the one that only needs handling.

CASE STATEMENT AND CASE EXPRESSION

A variation of CASE construct allows you to use it for output / assigning values. (pretty neat)

evaluation := 
  CASE
    WHEN grade BETWEEN 90 AND 100 THEN 'very good'
    WHEN grade BETWEEN 80 AND 89  THEN 'good'
    WHEN grade BETWEEN 70 AND 79  THEN 'fair'
    ELSE 'fail'
  END;


CASE_NOT_FOUND

this exception object is thrown when you don't provide an ELSE case and no case handles the given parameter passed to the statement.

(TODO: expound)