Matthew Wilson wrote:
> I'm trying to comprehend how NULL values interact with unique indexes.
> It seems like I can insert two rows with NULL values in a column with a
> unique constraint just fine.
> Is there something special about NULL? Can anyone post some links to
> explain what is going on?
Last paragraph just above 5.3.4.
> What's the deal with NULL?
NULL = NULL is not true, it's null
NULL <> NULL is not false, it's null
It's the normal SQL 3 state logic (true/false/null) with only the true
value from the comparison causing the constraint violation. Think of
the unique constraint check like "does this value equal any other value
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