The Case of the Confusing CASE

This odd little piece of code was featured in the weekly PL/SQL Challenge quiz 12 March - 18 March 2016.

What do you think will be displayed after executing the following block?

DECLARE
   my_flag   BOOLEAN;
BEGIN
   CASE my_flag
      WHEN my_flag IS NULL
      THEN
         DBMS_OUTPUT.PUT_LINE ('my_flag is NULL');
      WHEN TRUE
      THEN
         DBMS_OUTPUT.PUT_LINE ('my_flag is TRUE');
      ELSE
         DBMS_OUTPUT.PUT_LINE ('my_flag is FALSE');
   END CASE;
END;
/

At first glance (if you are like me), you would say "my_flag is NULL", right?

After all, my_flag is initialized to NULL when declared, and I don't change the value.

But, lo and behold, you will see:

my_flag is FALSE

Curious, right?

So what's going on? Well, we have a very confused and confusing piece of code: I have written a simple CASE (which is of the form CASE expression WHEN ...), but then my WHEN clauses follow a typical searched CASE format (CASE WHEN expr1 ... WHEN expr2 ...).

CASE is a really wonderful feature in PL/SQL (and many other languages, of course), but you need to make sure you use it properly.