1. In normalization theory of relation database, finding candidate key and primary attribute is always a difficult problem.

2. Finally, an algorithm to find the candidate key, whose cardinality is M, based on the attributes analysis table, is given.

3. Seeking for candidate key of relation scheme is an important question in designing database theory.

4. Seeking the candidate key of the relation scheme is an important procedure in normalizing relation.

5. No non-prime attribute in the table is functionally dependent on a part (proper subset) of a candidate key.

6. Cryptography can do nothing in resolving above problems, but digital watermarking which is one of the research focus in the academic world presents a candidate key to those questions.

7. Finally, a polynomial - time algorithm to find the candidate key whose cardinality is M is given.

8. Candidate keys from conversion rules sometimes are different from candidate keys from definition of candidate key in the process of converting Entity-Relationship Diagram into Relation Model.