Could you help me with Database Management Systems & Theory

a. What restrictions apply to the use of the aggregate functions within the SELECT statement? How do nulls affect the aggregate functions? b. What is the difference between a subquery and a join? Under what circumstances would you not be able to use a subquery? c. What is the result of an attempt to add, subtract, multiply, or divide two number fields, one of which contains a null value? d. Explain why the two following commands produce different results. SELECT DISTINCT COUNT (V_CODE) FROM PRODUCT; SELECT COUNT (DISTINCT V_CODE) FROM PRODUCT; e. Consider the relation instance EXAM (Student, Subject, Rank). The meaning of an EXAM tuple is that a specified student examined in a specified subject achieves a specified rank in the class. Further, no two students obtain the same rank in the same subject. EXAM Figure 1 What is (are) the candidate key(s) of EXAM? Based on your primary key, what normal form violations exist in EXAM