3.15 If (A, B) →C, then can we also say that A →C? ANSWER：No, neither A nor B will determine C by itself. 3.16 If A → (B, C), then can we also say that A →B? ANSWER：Yes, it is true that A →B and A →C. 3.19 If it is true that―PartNumber → PartWeight‖. Does that mean that PartNumber will be unique in a relation?
ANSWER：No, maybe there are other objects to determine the PartWeight. 3.20 Under what conditions will a determinant be unique in a relation?
ANSWER： A determinant is unique in a relation only if it determines every other column in the relation. 3.21 What is the best test for determining whether a determinant is unique?
ANSWER：The best strategies are to think about the nature of the business activity from which the data arise and to ask the users. 3.22 What is a composite key?
ANSWER：Keys that have two columns or more are called composite keys. 3.23 What is a candidate key?
ANSWER：A candidate key is a determinant that determines all of the other columns in a relation. 3.24 What is a primary key? ANSWER：When designing a database, one of the candidate keys is selected to be the primary key. The DBMS will use it as its primary means for finding rows in a table. 3.25 Explain the difference between a candidate key and a primary key.
ANSWER：A table has only one primary key, but it can has many candidate keys. 3.26 What is a surrogate key?
ANSWER：A surrogate key is an artificial column that is added to a table to serve as the primary key. 3.27 Where does the value of a surrogate key come from?
ANSWER：The DBMS assigns a unique value to a surrogate key when the row is created. The assigned value never changes. 3.28 When would you use a surrogate key?
ANSWER：Surrogate keys are used when the primary key is large and unwieldy.
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3.29 What is a foreign key? ANSWER：A foreign key is a column or composite of columns that is the primary key of a table other than the one in which it appears. 3.30 The term domestic key is not used. If it were used, however, what do you think it would mean?
ANSWER：A domestic key is a column or composite of columns that is only the primary key of a table in which it appears. 3.31 What is a normal form? ANSWER：Normal form is a collection of relational patterns that conform to a certain level of a certain level. 3.33 Explain why duplicated data leads to data integrity problems.
ANSWER：Any table that has duplicated data is susceptible to update anomalies. 3.34 What relations are in 1NF?
ANSWER：Any table meeting the conditions of all the characteristics of relations is a relation in 1NF. 3.35 Which normal forms are concerned with functional dependencies?
ANSWER：BCNF, 3NF and 2NF are all concerned with functional dependencies. 3.36 What conditions are required for a relation to be in 2NF?
ANSWER：It requires a database table or a row for each instance must be uniquely distincted. To achieve the requirement, it usually requires a column to store the unique identification for each instance. 3.37 What conditions are required for a relation to be in 3NF?
ANSWER：It requires a database table does not include non-primary key information is already contained in other tables. 3.38 What conditions are required for a relation to be in BCNF? ANSWER：
(1)All non-primary attributes are totally dependent function keys for each candidate. (2)All the main attributes are also completely functional dependency for each candidate key that does not contain them.
(3)It does not have any property fully functionally dependent on any one set of properties of non-candidate keys. 3.39 If a relation is in BCNF, what can we say about it with regard to 2NF and 3NF?
ANSWER：If a relation is in BCNF, we can say it also in 3NF and 2NF.
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3.40 What normal form is concerned with multivalued dependencies? ANSWER：4NF is concerned with multivalued dependencies. 3.41 What is the premise of Fagin‘s work on DK/NF?(假设)
ANSWER：He supposed that a relation in DK/NF has no modification anomalies. 3.42 Summarize the three categories of normalization theory.
ANSWER：The first category is concerned with anomalies that are caused by functional dependencies. The second category is concerned with anomalies that are caused by multivalued dependencies. The third category is concerned with anomalies that are caused by data constraints and odd conditions.
3.43 In general, how can you transform a relation not in BCNF into ones that are in BCNF?
ANSWER：You can also design (or redesigning) the tables so that every determinant is a candidate key. 3.44 What is a referential integrity constraint? ANSWER：Referential integrity constraint is a set of integrity rules, which defines the semantic constraints to be observed by the data model. 数据完整性约束是一组完整性规则的集合，它定义了数据模型必须遵守的语义约束。 3.45 Explain the role of referential integrity constraints in normalization.（说明参照完整性约束在规范化中的作用）
ANSWER：To ensure that the data stored in the database is correct, effective and compatible. 以确保数据库中存储的数据正确、有效、相容 3.46 Why is an un-normalized relation like a paragraph with multiple themes? ANSWER：Because normalized relation like a paragraph with only one themes.
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