2. A relation is a set of ordered tuples, by definition. On the other hand, according to the definition of set, it does not contain duplicate value. Thus, a collection of tuples will be called relation only when there is no duplicates. So, there is no duplicate tuple in a relation. Primary key is used in relations to maintain this property. (Elmasri, 2008)3. Candidate key can uniquely identify a tuple from a relation. Any candidate key can do the same task. On the other hand primary key is a candidate key that is designated as the same for a relation. There are other use of primary key, for example, in referencing to the foreign key, indexing to support searching etc. If there is two or more designated primary key, then referencing, indexing etc. will be problematic, more computational resources will be consumed to maintain properties of primary key for both. So, it is not used in practice. (Elmasri, 2008)
Elmasri, R. (2008). Fundamentals of Database Systems (6th ed.). Pearson.
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