Hi,
How can I go about having a cross-platform compatible compound unique
constraint over two columns where there can be only one record with NULL in
one column and a given value in the other?
I want something like this:
UniqueConstraint("parent_id", "name")
except "parent_id" is nullable, and I want to disallow creating multiple
records with parent_id=NULL and the same name. The problem here is that
some database management systems, PostgreSQL for example, will treat all
NULL values as not-equal inside a unique constraint, which makes sense for
a single column unique, but not really for a multi-column one. This will
allow multiple records with the same name and parent_id=NULL again.
In PostgreSQL, apparently I would need to create a unique index in addition
to the constraint. I would really like to solve this without writing
database-specific code, though.
Does anybody have a good solution for that?
Guido
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