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Re: Question on id constraints
- From: Laura Hatcher <lhatcher@a...>
- To: "Anderson, John" <John@B...>
- Date: Tue, 31 Jul 2001 13:19:04 -0400
1. It is not necessary for the pk to be defined earlier in the schema
file according to an example from the W3C. In the example that they
give they create a keyref that refers to another labeled "state" which
doesn't occur until much later.
<xs:keyref name="vehiclestate" refer="state">
<xs:element ref="state" maxOccurs="unbounded"/>
I would assume that the reasoning for this is that the key must be defined
within context that it is in and not within a special order. If there
is a key with respect to an element then it must be defined with in that
element definition. If the ordering mattered then that would limit
the potency of xml. Say for example a element has a key reference
to another key within another element and that element has a key reference
to a key within the first element, then there would be no way to define
2. No it is not required for pk to be defined within the same
element as the keyref. If you think of the element definition as
a database table and the pk representing a primary key, then it wouldn't
make sense for you to define a key reference to that key within the same
table. Instead the more likely use of key references is to refer
to a key within another table (or element within this case).
I hope that this information helped,
"Anderson, John" wrote:
I'm trying to decipher the schema spec
on identity contstraint validity, but perhaps someone can save me a few
hours and a few aspirin.
If I have:
<keyref name="ref" refers="pk"
1. Is the key "pk" required to be defined
physically earlier in the schema file (I would have thought not)
2. Is pk required to be defined on
the same element as ref, or on a descendant or anscetor of it, or just
anywhere in the same schema?
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