You can always add a constraint that these columns cannot be NULL
themselves. That removes this problem.
On Sep 28, 2008, at 11:17 PM, Klint Gore <[EMAIL PROTECTED]> wrote:
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?
http://www.postgresql.org/docs/8.3/interactive/ddl-constraints.html#AEN2058
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 already recorded".
klint.
--
Klint Gore
Database Manager
Sheep CRC
A.G.B.U.
University of New England
Armidale NSW 2350
Ph: 02 6773 3789 Fax: 02 6773 3266
EMail: [EMAIL PROTECTED]
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