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, .NET, C# and Firebird expert, focusing on data and business layers, language constructs, parallelism, databases and performance. He's Microsoft Most Valuable Professional and frequent speaker. You can read his articles, guides and tips and tricks at www.tabsoverspaces.com.