Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 9: SQL Server Storage and Index Structures
Sparse columns largely fall under the heading of “advanced topic,” but I do want you to know they are
there and can be a viable solution for particular scenarios.
Understanding Indexes
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Webster’s dictionary defines an index as:
A list (as of bibliographical information or citations to a body of literature) arranged usually in alphabetical
order of some specified datum (as author, subject, or keyword).
I’ll take a simpler approach in the context of databases and say it’s a way of potentially getting to data a
heck of a lot quicker. Still, the Webster’s definition isn’t too bad — even for our specific purposes.
Perhaps the key thing to point out in the Webster’s definition is the word “usually” that’s in there. The
definition of “alphabetical order” changes depending on a number of rules. For example, in SQL Server,
we have a number of different collation options available to us. Among these options are:
❑ Binary — Sorts by the numeric representation of the character (for example, in ASCII, a space is
represented by the number 32, the letter “D” is 68, and the letter “d” is 100). Because everything
is numeric, this is the fastest option. Unfortunately, it’s not at all the way in which people think,
and can also really wreak havoc with comparisons in your WHERE clause.
❑ Dictionary order — This sorts things just as you would expect to see in a dictionary, with a
twist. You can set a number of different additional options to determine sensitivity to case,
accent, and character set.
It’s fairly easy to understand that if we tell SQL Server to pay attention to case, then “A” is not going to
be equal to “a.” Likewise, if we tell it to be case insensitive, then “A” will be equal to “a.” Things get a
bit more confusing when you add accent sensitivity. SQL Server pays attention to diacritical marks, and
therefore “a” is different from “á,” which is different from “à.” Where many people get even more confused
is in how collation order affects not only the equality of data, but also the sort order (and, therefore,
the way it is stored in indexes).
By way of example, let’s look at the equality of a couple of collation options in the following table, and
what they do to our sort order and equality information:
Collation Order Comparison Values Index Storage Order
Dictionary order, case insensitive,
accent insensitive (the default)
Dictionary order, case insensitive,
accent insensitive, uppercase
preference
Dictionary order, case sensitive A ≠ a, Ä ≠ ä, Å ≠ å,
a ≠ à ≠ á ≠ â ≠ ä ≠ å,
A ≠ Ä ≠ Å
A = a = à = á = â = Ä = ä = Å = å a, A, à, â, á, Ä, ä, Å, åt
A = a = à = á = â = Ä = ä = Å = å A, a, à, â, á, Ä, ä, Å, å
A, a, à, á, â, Ä, ä, Å, å