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Sybase and MS SQL Server differ from other RDBMS
types in the way NULL values are handled. This can have an effect
when writing constraints. (e.g. if you want to port).
On these RDBMS types, the
following is sufficient to ensure that a constraint is only
evaluated in case of UPDATES:
WHERE OLD(VALUE) != VALUE
However, for transparency you
are advised to write:
WHERE OLD(VALUE) != VALUE
AND OLD(KEY) IS NOT NULL
AND KEY IS NOT NULL
The first AND clause ensures
that INSERTS are excluded, the second AND clause ensures that
DELETES are excluded.
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