Zombie SQL


October 31, 2018 by Kenneth Fisher

It Halloween so time for a scary SQL story. Ok, maybe not that scary. Ok, not scary at all, but still a bad practice. It’s Zombie code so it seemed like something fun to talk about on Halloween. But what is it?

Zombie Code

Code that is left in place but commented out.

At least that’s the best definition I was able to find. You’ve seen it right? Those big queries left in the stored procedure but commented out? Or lines of the WHERE clause commented out? Heck, I’ll bet you’ve done it. I know I have.

“I might need that code later.”
“That was pretty cool, I’ll save it so I can look at it again.”
“I just don’t like deleting code if I don’t have to.”
“It’s not working, I’ll comment it out and come back to it later.”

The problem is, zombie code is distracting. It makes it harder to read what’s actually going on, and as you learn to tune it out, you are also learning to tune out the real comments. And comments are important.

CREATE PROCEDURE ReallyImportantBatchProcess AS

SELECT ReallyImportantColumn1, ReallyImportantColumn2, 
	ReallyImportantColumn3, ReallyImportantColumn4
	INTO #ImportantToday
FROM ReallyImportantData
WHERE ImportantDate = Today;

                      /|x x|
                     /\( - )
            /=`_'-'-'/  !!
            |-{-_-_-}     !
            (-{-_-_-}    !
             \{_-_-_}   !
          ____%%@ @%%_______

-- [¬º-°]¬
ALTER TABLE #ImportantToday ADD Column Meh bit

UPDATE #ImportantToday SET Meh = 1 /* Did you see this? */
-- WHERE ReallyImportantColumn3 = 74

Now to be fair, when you are in the middle of development it can be perfectly reasonable to leave bits of code commented out. If it’s a personal script then I’d expect bits of commented code to expand the usefulness of the script. However, as code moves from development to test, and in particular when it moves past test into model office or even production, then all of that commented code should be removed. In the days of version control software, there is very little reason to leave bits of code commented out just so you can see them later and leaving code so it can be uncommented to add functionality shouldn’t be happening in production. The only comments that you want to leave in your code are the important ones. The ones that actually add value. That way when people scan the code, the comments jump out, they aren’t just part of the hoard of zombies.

Happy Halloween

5 thoughts on “Zombie SQL

  1. Roland Alexander says:

    We’ve gone so far as to add this to the coding standards, and require that legacy code be cleaned up if it’s touched. Maybe you should write a companion piece on “Vampire SQL” – comments that don’t tell you anything useful but still distract you: such as “select results from temp table”. Um, duh…there’s the SELECT right there…

  2. […] Usually if you need to disable a single trigger it’s for something like a load, or maybe you just don’t need it anymore and feel like creating some zombie code. […]

  3. Joe Celko says:

    I found that “zombie code” can be very handy. Over the decades that I’ve been writing SQL, I found that it can be very useful. I have had the habit when it was on contract of writing for the next release of the SQL product I’m using. For example, when Microsoft was finally getting rid of the old Sybase extended equality (*=) notation and syntax in favor of the ANSI/ISO standard infixed join operations ( [INNER | LEFT OUTER| RIGHT OUTER | FULL OUTER] JOIN), I would use the extended equality, but comment out the ANSI/ISO standard syntax. With Microsoft finally caught up with the standards, I told my clients to remove the comments from around the new notation and delete the old stuff. While this cost me some extra consulting work, it made me a real hero.

    • I can see that. Of course I’d almost call that the reverse of zombie code since it’s writing code that you plan on using in the future. Maybe “prenatal code”?

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