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+
+
+
+
+2023年9月8日 
+
+星期五 
+
+概念模型 CDM、
+
+逻辑模型 LDM 、
+
+物理模型 PDM
+
+ 三要素:1、实体:画矩形.里面写实体的各称 加下划线.
+ 属性:椭圆或圆形,里面写属性名称,用线条与实体直连,如果是主要性.
+关系:用线条将两个实体相连,中间可以用菱形来表示之间联系,菱形可以当实付，也可有自己属性.
+
+```sql
+/*==============================================================*/
+/* DBMS name:      MySQL 5.0                                    */
+/* Created on:     2023/9/10 22:21:34                           */
+/*==============================================================*/
+
+
+drop table if exists Borrowing;
+
+drop table if exists UserId;
+
+drop table if exists book;
+
+drop table if exists dengji;
+
+drop table if exists jiyie;
+
+drop table if exists "return";
+
+/*==============================================================*/
+/* Table: Borrowing                                             */
+/*==============================================================*/
+create table Borrowing
+(
+   brtime               char(10) not null,
+   userid               int(10) not null auto_increment,
+   username             char(6) not null,
+   returntime           date not null,
+   bokname              char(5) not null,
+   bokid                int(10) not null auto_increment,
+   primary key (userid, bokid)
+);
+
+/*==============================================================*/
+/* Table: UserId                                                */
+/*==============================================================*/
+create table UserId
+(
+   user_id              int(10) not null auto_increment,
+   user_name            char(5) not null,
+   user_password        char(6) not null,
+   primary key (user_id)
+);
+
+/*==============================================================*/
+/* Table: book                                                  */
+/*==============================================================*/
+create table book
+(
+   bk_id                int(10) not null auto_increment,
+   bk_name              char(10) not null,
+   primary key (bk_id)
+);
+
+/*==============================================================*/
+/* Table: dengji                                                */
+/*==============================================================*/
+create table dengji
+(
+   bk_id                int not null,
+   user_id              int not null,
+   primary key (bk_id, user_id)
+);
+
+/*==============================================================*/
+/* Table: jiyie                                                 */
+/*==============================================================*/
+create table jiyie
+(
+   userid               int not null,
+   bokid                int not null,
+   user_id              int not null,
+   primary key (userid, bokid, user_id)
+);
+
+/*==============================================================*/
+/* Table: "return"                                              */
+/*==============================================================*/
+create table "return"
+(
+   userid               int not null,
+   bokid                int not null,
+   bk_id                int not null,
+   primary key (userid, bokid, bk_id)
+);
+
+alter table dengji add constraint FK_dengji foreign key (bk_id)
+      references book (bk_id) on delete restrict on update restrict;
+
+alter table dengji add constraint FK_dengji2 foreign key (user_id)
+      references UserId (user_id) on delete restrict on update restrict;
+
+alter table jiyie add constraint FK_jiyie foreign key (userid, bokid)
+      references Borrowing (userid, bokid) on delete restrict on update restrict;
+
+alter table jiyie add constraint FK_jiyie2 foreign key (user_id)
+      references UserId (user_id) on delete restrict on update restrict;
+
+alter table "return" add constraint FK_return foreign key (userid, bokid)
+      references Borrowing (userid, bokid) on delete restrict on update restrict;
+
+alter table "return" add constraint FK_return2 foreign key (bk_id)
+      references book (bk_id) on delete restrict on update restrict;
+
+
+
+```