Thanks all for your help, I have created a clustered primary key on the three columns and added 2 unique indexes on the 2 pairs and tested it.

we are reducing...

ChrisM@Work (11/16/2015)

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The first and second examples would violate the unique constraint you already have. Are you looking to replace your existing unique constraint on vhid, or implement the new rules...

Thanks, this has been very helpful.

however, what is the meaning of the "|" character around some parameters?

Also these parameters havent been defined anywhere so how are they used?

thanks