发布时间 : 星期三 文章数据库系统基础教程答案ch4更新完毕开始阅读
数据库系统基础教程答案ch4
4.1.1
4.1.2 a)
b)
c)
In c we assume that a phone and address can only belong to a single customer (1-m relationship represented by arrow into customer).
d)
In d we assume that an address can only belong to one customer and a phone can exist at only one address.
If the multiplicity of above relationships were m-to-n, the entity set becomes weak and the key ssNo of customers will be needed as part of the composite key of the entity set.
In c&d, we convert attributes phones and addresses to entity sets. Since entity sets often become relations in relational design, we must consider more efficient alternatives.
Instead of querying multiple tables where key values are duplicated, we can also modify attributes:
(i) Phones attribute can be converted into HomePhone, OfficePhone and CellPhone. (ii) A multivalued attribute such as alias can be kept as an attribute where a single column can be used in relational design i.e. concatenate all values. SQL allows a query \
4.1.3
4.1.4 a)