This chapter discusses normalization of relations when the candidate keys of a relation have missing information represented by nulls. The chapter shows that problems and confusion can arise in normalizing relations with nulls in candidate keys. Candidate keys with missing information commonly are found in relations that represent information on two entities with a one-to-one relationship between them. The current definition of Boyce-Codd Normal Form (BCNF) is ineffective in identifying poor designs in such relations that may have insertion/deletion anomalies. Domain Key Normal Form (DKNF) also suffers from the same problem. It is shown that the above problem can be corrected by incorporating the concept of entity integrity rule into the definitions of BCNF and DKNF. This chapter also shows that incorporating the entity integrity rule into the definition of either a relation or a candidate key does not provide a satisfactory solution to the problem.