[简答题]
【说明】
设有下列关于图书借阅系统的E—R图。图中矩形表示实体,圆表示属性,双圆表示关键字属性,菱形表示实体间的联系。假定已通过下列SQL语言建立了基本表:
CREATE TABLE Readers
(Rao
CHAR(6) PRIMARY KEY,
Rname CHAR(20) NOT
NULL,
Raddress CHAR(200),
Rphone CHAR(15));
CREATE TABLE
Books
(Brio CHAR(6) PRIMARY
KEY,
Rname CHAR(50) NOT NULL);
CREATE TABLE Administrators
(Ano CHAR(6) PRIMARY KEY,
Aname
CHAR(20) NOT NULL);
CREATE TABLE Borrows
(Rno CHAR(6) NOT NULL,
Bno CHAR(15) NOT NULL,
Ano CHAR(6)
NOT NULL,
Bdate DATE,
Rdate DATE,
PRIMARY
KEY(Rno,Bno,Ano),
FOREGIN KEY(Rno)
REFERENCE Readers(Rno),
FOREGIN KEY(Bno)
REFERENCE Book(Bno)
FOREGIN KEY(Ano)
REFERENCE Administrators(Ano));
CREATE TABLE
Readers
(Rao CHAR(6) PRIMARY
KEY,
Rname CHAR(20) NOT NULL,
Raddress CHAR(200),
Rphone CHAR(15));
CREATE TABLE Books