As Kent already says SQL Server 2005 doesn't support xsd:unique.

The only way I know to enforce uniqueness within the schema is to use the xs:ID
type. However you have to know that uniqueness is enforced at the "cell" level.
If you use have a table with one typed XML column, an xs:ID value that appears
in one row cannot appear in the same row but can appear in a different row.
If you have a table with multiple typed XML columns, an xs:ID value can appear
only once in each "cell" but can apear on the same row in different columns.

Here's a quick example.

CREATE XML SCHEMA COLLECTION SCID AS '
http://www.w3.org/2001/XMLSchema">












'
go

CREATE TABLE T (iCol int primary key, xmlCol XML(SCID))
go

INSERT INTO T VALUES (0, '')
-- XML Validation: ID constraint check failed. Found attribute named 'a' with
duplicate ID value 'ID000'. Location: /*:e[2]/@*:a

INSERT INTO T VALUES (1, '')
-- succeeds

INSERT INTO T VALUES (2, '')
-- succeeds; we reuse the same ID value but in a different row

UPDATE T SET xmlCol.modify('insert as first into /') WHERE iCol =
1
-- XML Validation: ID constraint check failed. Found attribute named 'a' with
duplicate ID value 'ID001'. Location: /*:e[2]/@*:a

I hope this helps

Denis Ruckebusch
http://blogs.msdn.com/denisruc
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