> Hallo Hugo, > > > When should I use a Unique Key instead of a Index? > > > > I know that a Unique Key does not allow duplicated values like a Primary > > Key, but what more? > > That, and it creates an index automatically. Also, if you want a column to be > referenced by a foreign key, it has to be either the PRIMARY KEY or UNIQUE.
Notice that since Firebird 1.5, a unique key constraint does allow multiple null instances in the column. http://www.firebirdsql.org/file/documentation/reference_manuals/reference_material/html/langrefupd25-ddl-table.html#langrefupd25-ct-unique-keys Cheers, Paul Vinkenoog
