From time to time I got a question about the difference between unique key and unique constraint. Both elements enforcing uniqueness across values in column(s). But the unique index cannot be used in referential integrity constraints. The unique index can be considered as a “tool” for unique constraint.
Jiří Činčura is an independent developer focusing on data and business layers, language constructs, parallelism and databases. Specifically Entity Framework, asynchronous and parallel programming, cloud and Azure. He's Microsoft Most Valuable Professional and you can read his articles, guides, tips and tricks at www.tabsoverspaces.com.