What is the difference between controlled and uncontrolled redundancy? Illustrate with examples.   What are the main reasons for and potential advantages of distributed ­databases?   Describe the two alternatives for specifying structural constraints on relationship types. What are the advantages and disadvantages of each?   Can an identifying relationship of a weak entity type be of a degree greater than two? Give examples to illustrate your answer.    4.1. What is a subclass? When is a subclass needed in data modeling?   4.7. What is the difference between a specialization hierarchy and a specialization lattice?