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+## 心得
+
+1. 概念模型COM,ER图。
+2. 逻辑模型LDM,计算机。
+3. 物理模型PDM，从具体数据系角度
+4. 生成DDL
+
+```mysql
+/*==============================================================*/
+/* DBMS name:      MySQL 5.0                                    */
+/* Created on:     2023/9/10 23:25:52                           */
+/*==============================================================*/
+CREATE DATABASE library charset utf8;
+use library;
+
+drop table if exists b_type;
+
+drop table if exists book;
+
+drop table if exists borrow;
+
+drop table if exists class;
+
+drop table if exists reader;
+
+drop table if exists readertype;
+
+drop table if exists "return";
+
+drop table if exists student;
+
+drop table if exists teacher;
+
+/*==============================================================*/
+/* Table: b_type                                                */
+/*==============================================================*/
+create table b_type
+(
+   t_id                 int not null auto_increment,
+   t_name               varchar(10) not null,
+   t_decription         varchar(20) not null,
+   primary key (t_id)
+);
+
+/*==============================================================*/
+/* Table: book                                                  */
+/*==============================================================*/
+create table book
+(
+   b_id                 int not null auto_increment,
+   t_id                 int not null,
+   author               varchar(10) not null,
+   b_name               varchar(20) not null,
+   publishing           varchar(20) not null,
+   price                int not null,
+   b_num                int not null,
+   primary key (b_id)
+);
+
+/*==============================================================*/
+/* Table: borrow                                                */
+/*==============================================================*/
+create table borrow
+(
+   b_id                 int not null,
+   r_id                 varchar(10) not null,
+   bor_time             time not null,
+   bor_num              int not null,
+   primary key (b_id, r_id)
+);
+
+/*==============================================================*/
+/* Table: class                                                 */
+/*==============================================================*/
+create table class
+(
+   c_id                 int not null auto_increment,
+   c_name               varchar(10) not null,
+   c_num                int not null,
+   primary key (c_id)
+);
+
+/*==============================================================*/
+/* Table: reader                                                */
+/*==============================================================*/
+create table reader
+(
+   r_id                 varchar(10) not null,
+   rt_id                int not null,
+   r_name               varchar(5) not null,
+   state                varchar(3) not null,
+   primary key (r_id)
+);
+
+/*==============================================================*/
+/* Table: readertype                                            */
+/*==============================================================*/
+create table readertype
+(
+   rt_id                int not null auto_increment,
+   rt_name              char(2) not null,
+   b_quantity           int not null,
+   b_time               int not null,
+   primary key (rt_id)
+);
+
+/*==============================================================*/
+/* Table: "return"                                              */
+/*==============================================================*/
+create table "return"
+(
+   r_id                 varchar(10) not null,
+   b_id                 int not null,
+   ret_time             time not null,
+   ret_num              int not null,
+   primary key (r_id, b_id)
+);
+
+/*==============================================================*/
+/* Table: student                                               */
+/*==============================================================*/
+create table student
+(
+   s_id                 int not null auto_increment,
+   rt_id                int not null,
+   c_id                 int not null,
+   s_sex                char(1) not null,
+   s_name               varchar(5) not null,
+   primary key (s_id)
+);
+
+/*==============================================================*/
+/* Table: teacher                                               */
+/*==============================================================*/
+create table teacher
+(
+   te_id                int not null auto_increment,
+   rt_id                int,
+   te_name              varchar(10) not null,
+   te_sex               char(1) not null,
+   primary key (te_id)
+);
+
+alter table book add constraint FK_classify foreign key (t_id)
+      references b_type (t_id) on delete restrict on update restrict;
+
+alter table borrow add constraint FK_borrow foreign key (b_id)
+      references book (b_id) on delete restrict on update restrict;
+
+alter table borrow add constraint FK_borrow2 foreign key (r_id)
+      references reader (r_id) on delete restrict on update restrict;
+
+alter table reader add constraint FK_ׁ֟ؖ` foreign key (rt_id)
+      references readertype (rt_id) on delete restrict on update restrict;
+
+alter table "return" add constraint FK_return foreign key (r_id)
+      references reader (r_id) on delete restrict on update restrict;
+
+alter table "return" add constraint FK_return2 foreign key (b_id)
+      references book (b_id) on delete restrict on update restrict;
+
+alter table student add constraint FK_Relationship_4 foreign key (rt_id)
+      references readertype (rt_id) on delete restrict on update restrict;
+
+alter table student add constraint FK_Relationship_6 foreign key (c_id)
+      references class (c_id) on delete restrict on update restrict;
+
+alter table teacher add constraint FK_Relationship_7 foreign key (rt_id)
+      references readertype (rt_id) on delete restrict on update restrict;
+
+
+```
+