The latest articles on DEV Community by dengzhu-hub (@dengzhuhub). https://dev.to/dengzhuhub https://media2.dev.to/dynamic/image/width=90,height=90,fit=cover,gravity=auto,format=auto/https:%2F%2Fdev-to-uploads.s3.amazonaws.com%2Fuploads%2Fuser%2Fprofile_image%2F1087315%2Fb26ae78b-4d12-4e75-802b-8565266e33c0.jpeg https://dev.to/dengzhuhub en dengzhu-hub Mon, 18 Mar 2024 06:55:16 +0000 https://dev.to/dengzhuhub/this-is-the-mysql-note-for-my-test-22pf https://dev.to/dengzhuhub/this-is-the-mysql-note-for-my-test-22pf <h1> <strong>MySQL笔记</strong> </h1> <h3> <strong>数据库系统特点</strong> </h3> <ul> <li>采用数据模型实现数据结构化</li> <li>数据冗余度小，实现数据共享，避免数据的不一致性</li> <li>较高的数据独立性</li> <li>数据有DBMS统一管理控制 <ul> <li>安全性</li> <li>完整性</li> <li>并发控制</li> <li>数据库恢复</li> </ul> </li> </ul> <h3> <strong>数据模型</strong> </h3> <h3> <strong>模型</strong> </h3> <p>现实世界特征的模拟和抽象</p> <h3> <strong>数据模型</strong> </h3> <p>现实世界数据特征的模拟和抽象</p> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img/20220907090408.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img/20220907090408.png</a></p> <h2> <strong>现实世界中客观对象的抽象过程</strong> </h2> <h3> <strong>数据模型的分类</strong> </h3> <ol> <li>概念数据模型</li> <li>逻辑数据模型 <ol> <li>层次</li> <li>网状</li> <li>关系</li> </ol> </li> <li>物理数据模型</li> </ol> <h3> <strong>数据模型的组成要素</strong> </h3> <ul> <li>数据结构 <ul> <li>是对系统静态特性的描述</li> <li>数据本身及数据之间的描述</li> </ul> </li> <li>数据操作 <ul> <li>是对系统动态特性的描述</li> </ul> </li> <li>数据的约束条件</li> </ul> <h3> <strong>实体联系数据模型</strong> </h3> <h3> <strong>实体</strong> </h3> <ul> <li>客观存在并可相互区别的个体。</li> <li>实体特性:描述实体的主要特征。</li> <li>实体集:具有相同特性实体的集合。</li> <li>实体标识符:唯一能确定实体集中某个实体的 最小实体特性集。</li> </ul> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img/20220907093255.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img/20220907093255.png</a></p> <p>计算机中的信息必须数字化。</p> <h3> <strong>E-R图</strong> </h3> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img/20220907095002.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img/20220907095002.png</a></p> <h3> <strong>关系模型</strong> </h3> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209140921207.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209140921207.png</a></p> <h3> <strong>数据约束</strong> </h3> <ul> <li>实体完整性</li> <li>参照完整性</li> <li>用户定义完整性</li> </ul> <h3> <strong>优缺点</strong> </h3> <p>优点：坚实的理论基础</p> <p>表达能力强</p> <p>简单</p> <p>数据独立性高</p> <p>缺点：效率低</p> <h3> <strong>数据库系统结构</strong> </h3> <h3> <strong>模式（Schema）</strong> </h3> <p>关于数据库和表的布局及特性的信息</p> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209140929691.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209140929691.png</a></p> <ul> <li>模式（）</li> </ul> <h3> <strong>关系模型</strong> </h3> <h3> <strong>域</strong> </h3> <h3> <strong>笛卡尔积</strong> </h3> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151421667.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151421667.png</a></p> <h3> <strong>码</strong> </h3> <ul> <li>候选码：在一个关系中，能够唯一标识元祖的属性或最小属性集称为关系的候选码</li> <li>主码：若一个关系中有多个候选码，则选择其中的一个为主码</li> <li>外码：</li> </ul> <h3> <strong>关系的性质</strong> </h3> <p>1） 分量必须取原子值</p> <p>2）列是同质的</p> <p>3）表中得列成为属性，不能重名</p> <p>4）列的顺序无关</p> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151456469.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151456469.png</a></p> <h3> <strong>关系完整性约束</strong> </h3> <ul> <li>实体完整性</li> <li>参照完整性</li> <li>用户自定义完整性</li> </ul> <h3> <strong>关系代数</strong> </h3> <ul> <li>概述</li> </ul> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151521002.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151521002.png</a></p> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151524605.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209151524605.png</a></p> <h3> <strong>选择运算</strong> </h3> <h3> <strong>选择</strong> </h3> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209221410745.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209221410745.png</a></p> <h3> <strong>投影</strong> </h3> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209221420156.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202209221420156.png</a></p> <h3> <strong>查询优化</strong> </h3> <p>!<a href="https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202210050854805.png">https://makeforpicgo.oss-cn-chengdu.aliyuncs.com/img202210050854805.png</a></p> <ul> <li>计算树</li> </ul> <p>ARE YOU KIDDING M</p> <p><a href="https://www.notion.so/Life-Wiki-68e9ff711b3c4deaa95a4ef860244fdd?pvs=21">Life Wiki</a></p> <p>泛关系模式</p> <p>决定因素：</p> <p>若x-y，则x叫决定因素</p> <p>平凡的函数依赖</p> <p>x-y，y 🤏🏻x，则x ➡️ y是平凡的函数依赖</p> <p>完全函数依赖</p> <p>部分函数依赖</p> <p>传递函数依赖</p> <p><a href="https://github.com/dengzhu-hub/for_picgo">https://github.com/dengzhu-hub/for_picgo</a></p> <h1> 第一范式（1NF） </h1> <div class="highlight js-code-highlight"> <pre class="highlight plaintext"><code>const greeting = "I'm a good example"; </code></pre> </div> <div class="highlight js-code-highlight"> <pre class="highlight plaintext"><code>const greeting = "I'm a bad example"; </code></pre> </div> webdev javascript programming beginners