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Short Substrings and Computed Columns

Erik Darling gives us a story about computed columns that turns out not to be about computed columns at all (having thereby subverted our expectations):

The problem is that when I tried to index it:

    ON dbo.Users(DisplayNameComputed);

I got this error:

Msg 537, Level 16, State 3, Line 21
Invalid length parameter passed to the LEFT or SUBSTRING function.

And when I tried to select data from the table, the same error.

Click through to find the real query killer.