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=================
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FreeTDS and dates
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=================
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Explanation of how pymssql and FreeTDS can break dates.
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Summary
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=======
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Make sure that FreeTDS is compiled with ``--enable-msdblib`` ``configure``
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option, or your queries will return wrong dates -- ``"2010-00-01"`` instead of
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``"2010-01-01"``.
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Details
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=======
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There's an obscure problem on Linux/\*nix that results in dates shifted back by
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1 month. This behaviour is caused by different ``dbdatecrack()`` prototypes in
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*Sybase Open Client DB-Library/C* and the *Microsoft SQL DB Library for C*. The
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first one returns month as 0..11 whereas the second gives month as 1..12. See
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this `FreeTDS mailing list post`_, `Microsoft manual for dbdatecrack()`_,
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and `Sybase manual for dbdatecrack()`_ for details.
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FreeTDS, which is used on Linux/\*nix to connect to *Sybase* and *MS SQL*
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servers, tries to imitate both modes:
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* Default behaviour, when compiled without ``--enable-msdblib``, gives
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``dbdatecrack()`` which is Sybase-compatible,
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* When configured with ``--enable-msdblib``, the ``dbdatecrack()`` function is
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compatible with *MS SQL* specs.
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pymssql requires *MS SQL* mode, evidently. Unfortunately at runtime we can't
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reliably detect which mode FreeTDS was compiled in (as of FreeTDS 0.63). Thus at
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runtime it may turn out that dates are not correct. If there was a way to detect
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the setting, pymssql would be able to correct dates on the fly.
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If you can do nothing about FreeTDS, there's a workaround. You can redesign your
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queries to return string instead of bare date:
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.. code-block:: sql
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SELECT datecolumn FROM tablename
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can be rewritten into:
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.. code-block:: sql
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SELECT CONVERT(CHAR(10),datecolumn,120) AS datecolumn FROM tablename
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This way SQL will send you string representing the date instead of binary date
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in datetime or smalldatetime format, which has to be processed by FreeTDS and
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pymssql.
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.. _FreeTDS mailing list post: http://lists.ibiblio.org/pipermail/freetds/2002q3/008336.html
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.. _Microsoft manual for dbdatecrack(): http://msdn.microsoft.com/en-us/library/aa937027(SQL.80).aspx
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.. _Sybase manual for dbdatecrack(): http://manuals.sybase.com/onlinebooks/group-cnarc/cng1110e/dblib/@Generic__BookTextView/15108
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