关系模式也遵循数据库的三级体系结构，称为(49)，(50)和(51)。(49)一的定义包括：模式名，属性名，值域名以及模式的主键。由于不涉及到物理存储方面的描述，仅仅是对数据本身的一些特性描述。(50)定义语言还需定义用户对数据进行操作的权限，例如是否允许读、修改等可来源于多个模式，(51)是作为文件看待的，每个元组就是一个记录。由于关系模式有键，因此一个关系可用散列方法或索引方法实现。如果关系的元组数目较少(100个以内)，那么也可以用“堆文件”方式实现(即没有特定的次序)。

(64)

(63)

(62)

(61)

(59)

In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

(57)

(58)

(56)

In(66)the strength of the carrier signal is varied to represent binary 1 or 0.(67)is a system that can map a name to an address and conversely an address to an name.(68)is a three -way handshaking authentication protocol that it's packets are encapsulated in the point - to -point protocol.(69)is an approach that enables an ATM Switch to behave like a LAN switch.(70)is an guided media which is made of glass or plastic and transmits signals in the form. of light.