The logic of three-valued logic

In my previous post, I explained what NULL does and does not mean, how and why the rule of NULL propagation forces any expression involving NULL to result in NULL. I also mentioned that comparisons involving NULL do not result in NULL but, rather, in a new “truth value” called “Unknown”. But there’s a lot more to be written about Unknown and it’s effect on logical expressions, and that’s exactly what this blog post (and the next one as well) will be about.   Most software developers will be intimately familiar with what is popularly known as Boolean logic but…
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NULL – The database’s black hole

I once read a scientific article on black holes that started of on the observation that, since a hole is the absence of anything and black is invisible in space, a black hole is in fact an invisible nothing – so what the heck are we talking about? Well, almost the same can be said about NULL in databases.   Let’s first take a look at what NULL is supposed to be. Here is the definition of NULL from the SQL-2003 standard:               null value: A special value that is used to indicate the absence of any data value.…
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The Bounding Box, corrected version

In a previous posting, I explained a minor problem in the “Bounding Box” and “Dynamic Bounding Box” algorithms I describe in chapter 9 of Adam Machanic’s book Expert SQL Server 2005 Development. In short, the problem is that I calculated the boundaries of the search area just a tiny bit too small, introducing a (very small) chance of relevant data not being found. I also presented a quick and dirty fix that basically works by deliberately making the search area too big, sacrificing (a small bit of) performance for correctness. And I promised to keep trying to find a better…
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Going to Dublin

I still lack the time to blog about proper hardcore SQL stuff, but I do want to make a quick post to let you all know about a great conference that I’ll be attending and speaking at.   I’m talking about the Irish Microsoft Technology Conference, to be held in Dublin, Ireland later this week (June 7th, to be precise). I am particularly excited about this conference, not only because it marks my first speaking engagement outside the Netherlands, but also because (unlike some other conferences I have attended) there is actually a pretty decent number of SQL Server related…
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Correcting my mistake

Probably the worst thing that can happen to a budding technical book author, is to find an error, even if it’s a minor one, in the submitted material just after the deadline for corrections closes. Figures that it should happen to me, eh?   Back in November last year, Adam Machanic asked me to contribute a chapter on spatial data for his book, Expert SQL Server 2005 Development. I have cursed myself for accepting his proposal several times, but I must admit that now all the hard work is done and the book has finally hit the shelves, I am…
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Can you trust your constraints?

SQL Server allows you to temporarily disable your CHECK and FOREIGN KEY constraints. I would not recommend this as part of your regular production schedule, but there are certainly cases where this comes in handy.   One example is if you’re copying thousands of rows from another table, and you already know with 100% certainty that the data doesn’t violate any constraint. The overhead of checking could impact performance of your process; in the worst case making the execution time exceed your maintenance window. Disabling the constraints can result in a tremendous reduction of execution time.   Another example would…
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Back with a vengeance!

It’s been quite a while since my last blog post. Almost three months, in fact. In case you have been wondering if I was ill, dead, or abducted by aliens – not at all. The reality was far worse: Adam Machanic has coerced me into contributing a chapter on spatial data for his excellent book “Expert SQL Server 2005 Development”, due out in May (rather than the original plan to ship in March, possibly due to a certain author of a certain chapter not meeting any of his deadlines…).   Anyway, the chapter is finished and I now finally find…
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The Beatles versus the Stones – the explanation

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On December 31 of last year, I posted this brain teaser, promising to post the answer “in a few days”. Apparently, 15 is a few J.   In case you have forgotten what the puzzle was about and are too lazy to click the link above, the bottom line is that I created and populated two tables, with the same schema but different content. One held the first and last name of each of the Beatles, the other held first and last name of each of the Rolling Stones. I then queried both tables, using the same query in both…
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The Beatles versus the Stones

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Here’s a nice brain teaser, just before the end of the year. Despite the title, it is related to SQL Server, not to music!   A common misconception amongst SQL Server users is that a clustered index on a table will ensure that data is returned in the order implied by that index. I have lost count of the number of times I had to disprove this notion.   Of course, there are many cases where the rows returned by a query will be in the order of the clustered index. Here’s a quick illustration, using the lineup that The…
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The table scan from hell

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Greg Linwood, a fellow SQL Server MVP, has started a series of articles in which he attempts to prove that having a clustered index on each table is not a good practice. However, he has failed to include the effects of fragmentation into account, so I decided to run some tests for myself. One of those test had rather upsetting results.   First a bit of background, for those of you who have never read a book by Kalen Delaney. A table without clustered index (also known as a “heap”) can be read in two ways: using a bookmark lookup…
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