Skip to main content

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.

Comments

  1. Ganesh Sridhar Koteshwar (Manoj)March 23, 2016 at 5:55 AM

    At a first glance I thought it was something to do with boolean variable. Later, I realized it is the same for all datatypes. So, it actually happens whenever the element in the switch statement is null. Any comparison with null will not be evaluated hence it wall fall to catch all(else) condition. A good food for brain!! :)
    Thanks!!!

    ReplyDelete
  2. Michael "Misha" RosenblumMarch 23, 2016 at 7:18 AM

    This comment has been removed by the author.

    ReplyDelete
  3. Steven FeuersteinMarch 23, 2016 at 7:23 AM

    Misha, thanks for the added explanation. I should have done that myself, but I knew you would want to contribute. :-)

    You are absolutely right. The reason this "works" - no compile errors - is that I am "searching" for a Boolean value. Change the datatype of my_flag to, say, integer, and it's a no-go situation.

    Gee, I was going to insert an image here to show everyone, but I guess I can't do that. Dumb Blogger. :-(

    I will post it on my twitter feed - twitter.com/sfonplsql

    ReplyDelete
  4. Michael "Misha" RosenblumMarch 23, 2016 at 7:51 AM

    And even with INTEGER you can get into trouble (if you really try) :-)
    Something like this would still compile:


    DECLARE
    my_flag integer;
    BEGIN
    CASE my_flag = 1 -- now it is BOOLEAN :-)
    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;

    P.s. Lost my original post somehow...

    ReplyDelete
  5. Steven FeuersteinMarch 23, 2016 at 8:05 AM

    Not sure what happened to your earlier comment, Misha. Maybe Blogger has a bad where clause for its delete. Or my likely I clicked a link I shouldn't. Here, for the benefit of my wonderful readers, is what Misha wrote (unfortunately out of order now):

    And even more fun starts when you try set default values :-). For example, if you initialize my_flag with FALSE, the output will be "my_flag is NULL".

    But the story becomes pretty straightforward if you take into an account the following:
    - simple CASE statement doesn't evaluate against NULL as value. I.e. the following will always go to ELSE:
    case your_variable
    when NULL ....
    else ...
    end case

    - all conditions are really resulting in BOOLEAN value. That's why a search case format is tolerated and doesn't cause any syntax issues. Oracle would first validate the condition from your example and build a real simple CASE:
    CASE my_flag
    WHEN TRUE --- my_flag IS NULL
    THEN ...
    WHEN TRUE
    THEN ...
    ELSE ...
    END CASE;
    Now Oracle checks that the variable is NULL and falls into ELSE clause - as expected.

    I hope, it clarifies the issue (a bit :-) ).

    ReplyDelete

Post a Comment

Popular posts from this blog

Running out of PGA memory with MULTISET ops? Watch out for DISTINCT!

A PL/SQL team inside Oracle made excellent use of nested tables and MULTISET operators in SQL, blending data in tables with procedurally-generated datasets (nested tables).  All was going well when they hit the dreaded: ORA-04030: out of process memory when trying to allocate 2032 bytes  They asked for my help.  The error occurred on this SELECT: SELECT  *    FROM header_tab trx    WHERE (generated_ntab1 SUBMULTISET OF trx.column_ntab)       AND ((trx.column_ntab MULTISET             EXCEPT DISTINCT generated_ntab2) IS EMPTY) The problem is clearly related to the use of those nested tables. Now, there was clearly sufficient PGA for the nested tables themselves. So the problem was in executing the MULTISET-related functionality. We talked for a bit about dropping the use of nested tables and instead doing everything in SQL, to avoid the PGA error. That would, however require lots of wo...
4 comments
Read more

How to Pick the Limit for BULK COLLECT

This question rolled into my In Box today: In the case of using the LIMIT clause of BULK COLLECT, how do we decide what value to use for the limit? First I give the quick answer, then I provide support for that answer Quick Answer Start with 100. That's the default (and only) setting for cursor FOR loop optimizations. It offers a sweet spot of improved performance over row-by-row and not-too-much PGA memory consumption. Test to see if that's fast enough (likely will be for many cases). If not, try higher values until you reach the performance level you need - and you are not consuming too much PGA memory.  Don't hard-code the limit value: make it a parameter to your subprogram or a constant in a package specification. Don't put anything in the collection you don't need. [from Giulio Dottorini] Remember: each session that runs this code will use that amount of memory. Background When you use BULK COLLECT, you retrieve more than row with each fetch, ...
4 comments
Read more

PL/SQL 101: Three ways to get error message/stack in PL/SQL

The PL/SQL Challenge quiz for 10 September - 16 September 2016 explored the different ways you can obtain the error message / stack in PL/SQL. Note: an error stack is a sequence of multiple error messages that can occur when an exception is propagated and re-raised through several layers of nested blocks. The three ways are: SQLERRM - The original, traditional and (oddly enough) not currently recommended function to get the current error message. Not recommended because the next two options avoid a problem which you are unlikely  to run into: the error stack will be truncated at 512 bytes, and you might lose some error information. DBMS_UTILITY.FORMAT_ERROR_STACK - Returns the error message / stack, and will not truncate your string like SQLERRM will. UTL_CALL_STACK API - Added in Oracle Database 12c, the UTL_CALL_STACK package offers a comprehensive API into the execution call stack, the error stack and the error backtrace.  Note: check out this LiveSQL script if...
5 comments
Read more