Complex sentence: In database relational modeling and implementation, a unique key is a set of zero, one or more attributes, the value (s) of which are guaranteed to be unique for each tuple (row) in a relation.
Split into simpler sentences: In database relational modeling and implementation, a unique key is a set of zero, one, or more attributes. The value (s) of these attributes are required to be unique for each tuple (row) in a relation.