Michael J. Swart

Consider the Following

Did you know you can do science anywhere. Science is about learning how Nature behaves. Well what if we replace Mother Nature with SQL Server? SQL Server’s not as pretty, but we can still learn things. It’s 2012! When we wonder about things – even databases – we don’t have to guess.

The Question

To keep things simple, I’m going to follow the scientific method here and this is step one. The question. All good science starts with a question. The question itself doesn’t matter. I just want to talk about the process.

So I take the question from this DBA Stackexchange question: Why are NULLS sorted first?

Good question! The SQL Standard does not actually prescribe any behavior here (says Wikipedia). So the actual behavior is left up to the RDBMS vendor. The likely answer to the question is that NULLS are sorted that way because Microsoft had to pick something and they probably just made a choice based on what other RDBMS’s were doing or other implementation considerations. Or who knows, maybe they just flipped a coin.

But that leads me to this question: Are they always sorted first? If I reverse the order of a sort by using ORDER BY column DESC do Null values still show up first? If I look at the manual, Microsoft tells me “Null values are treated as the lowest possible values” so that ORDER BY column DESC will place Null values at the end.

But the way the stack overflow question is worded makes me wonder a little and I’d like to find out for sure. So to repeat:

“If I sort the results of a query using ORDER BY column DESC do Null values come last?”

Hypothesis / Prediction

I’m placing my bets with Microsoft’s documentation and so I answer YES.

I predict that any query written with “ORDER BY column DESC” will place the Null values at the end. If I’m wrong, then it would seem like null values always first is the behavior here (Note: NULLS FIRST is actually mentioned in the SQL Standards as an extension to the ORDER BY clause, but it’s not implemented by SQL Server. This also lets me believe I’m not crazy for wondering.)

Test

That’s simple enough:

select *
from (values (1),(2),(3),(null)) as v(value)
order by value desc

The prediction says that the fourth value in the results will be NULL. And tada! The prediction is correct.

Analysis

There’s nothing surprising here, but maybe it raises some new questions. Like:

• What do other vendors do? (Turns out they do the same. This lends weight to Microsoft picking this behavior because it was common practice)
• What’s up with this NULLS FIRST keyword?

There’s a publish part here. The default assumption (that Microsoft documentation is often correct) is still valid so there’s nothing surprising to publish. Probably not worth a blog post (despite what you’re reading).

Boiled Down

Normally the thought processes aren’t so structured, but as long as the question comes first and the guess is checked against experiment, it counts.

It seems like common sense. And maybe I could have picked a better example. Science helps satisfy our curiosity. But it also can save us from kidding ourselves. Without this kind of thinking we risk turning into superstitious pigeons. I once saw someone perform a task (release and renew his ip address) three times out of habit “just so it takes”. That way lies madness.