数据库中的数据模型
我学数据库(3)--数据库中的数据模型 本次随笔和随笔（4）、随笔（5）和随笔（6）主要是详细的了解了一下数据模型的相关知识，因为这是数据库系统的核心与基础，所以必须对数据模型有一个清楚的认识，数据模型呢本来就是抽象出来的，所以呢这里面都是些概念的
我学数据库(3)--数据库中的数据模型
本次随笔和随笔（4）、随笔（5）和随笔（6）主要是详细的了解了一下数据模型的相关知识，因为这是数据库系统的核心与基础，所以必须对数据模型有一个清楚的认识，数据模型呢本来就是抽象出来的，所以呢这里面都是些概念的东西，可能有些难以理解，但是也没有办法，既然想学习这门“艺术”，就必须了解这些，而且必须有深刻的认识。不多说了，转入正题吧，开始数据模型的学习 ，fighting！
数据模型，一个是数据一个是模型，所以得先对这两个名词进行了解。
数据是记录信息的按一定规则排列组合的物理符号。可以是数字、文字、图像，也可以是计算机代码。对信息的接收始于对数据的接收，对信息的获取只能通过对数据背景的解读。
模型，像飞机模型等具体模型，对我们来说再熟悉不过了。因为这些我们都见过真实的事物。模型是现实世界中某个对象特征的模拟和抽象，是用以分析问题的概念、数学关系、逻辑关系和算法序列的表示体系。
综上，数据模型（Data Model）是数据特征的抽象，用来分析数据问题的概念、数据之间的各种关系的表示体系，也就是说，是用来描述数据、组织数据和对数据进行操作的。选择一个数据模型不是那么的随便，不能随便的对某数据进行抽象后就拿来当模型，数据模型必须满足一定的要求才行：一是能比较真实的模拟现实世界，这里用的比较一词来形容，是因为模型就是模型，不可能完全真实的模拟世界；而是容易被人们理解才行，晦涩难懂，那就失去了抽象的意义；三是便于在计算机上实现，这一个条件我是这样理解的，如果让人脑去实现，恐怕得是爱因斯坦或者霍金那样的才行啊，人脑毕竟是受限的，这个层次上电脑要比人脑好使多了。但问题又出现了，什么数据模型能很好的全面的满足这三个条件呢？答案是现在还没有,也就是说，这个地方的数据模型可能在那个地方不再适用。所以必须针对不同的对象和应用，也只能采用不同的数据模型去满足要求。在数据库系统中也是这样做的，即在开发实施数据库应用系统的不同阶段使用不同的数据模型：概念模型、逻辑模型和物理模型。其实是首先描述是什么，紧接着是如何表示，最后就是如何实现了。说白了就是发现问题，香港空间，分析问题和解决问题的这么一个过程。
在描述问题的过程中用的是概念数据模型，也叫信息模型，这是面向数据库用户的实现世界的数据模型，主要用来描述世界的概念化结构，它使数据库的设计人员在设计的初始阶段，摆脱计算机系统及DBMS的具体技术问题，集中精力分析数据以及数据之间的联系等，与具体的DBMS无关。此模型是面向现实世界中的真实需求，这个模型构造的是否成功直接决定了相应的数据库应用系统的成败与否。
在如何表示的过程中用的是逻辑数据模型，这是用户从数据库看到的数据模型，是具体的DBMS所支持的数据模型，网站空间，如网状数据模型、层次数据模型、关系模型、面向对象模型和对象关系模型等。此模型既要面向用户，又要面向系统。之所以说面向用户是因为逻辑模型是由概念模型直接转换而来，在转换的过程中还会有更深层次的问题暴露出来，而这些问题是要回归到概念模型再次讨论；之所以面向系统，在前面说了，既然是模型了就要容易被计算机实现，而此模型就是在计算机系统的观点上对数据建模的，主要用于DBMS的实现。
在实现的过程中使用的是物理数据模型，这是描述数据在存储介质上的组织结构的数据模型它不但与具体的DBMS有关，而且还和操作系统以及硬件有关。每一种逻辑数据模型在实现时都有其对应的物理数据模型。DBMS为了保证其独立性与可移植性，大部分物理数据模型的实现工作由系统自动完成，而设计者只设计索引、聚集等特殊结构。此模型纯粹的面向系统的，从计算机的角度来，从最低层次对数据进行抽象。
接下来了解一下数据模型的共性。一般而言，数据模型是一组严格定义的概念的集合。这些概念精确地描述了系统的静态特征（数据结构）、动态特征（数据操作）和完整性约束条件，这就是数据模型的三要素，即数据模型的共性。
数据结构是所研究的对象类型的集合。这些对象是数据库的组成部分，数据结构指对象和对象间联系的表达和实现，是系统静态特征的描述，包括两个方面：一方面是关于数据对象本身的，比如类型、内容、性质；一方面是关于数据之间的联系，服务器空间，关于数据之间是如何相互联系的。
数据操作是对数据库中对象的实例允许执行的操作集合，主要指检索和更新（插入、删除、修改）两类操作。数据模型必须定义这些操作的确切含义、操作符号、操作规则（如优先级）以及实现操作的语言。数据操作是对系统动态特征的描述。
数据完整性约束是一组完整性规则的集合，规定数据库状态及状态变化所应满足的条件，以保证数据的正确性、有效性和相容性。原先也提过为什么文件系统已不能再满足与日递增的要求，其中一条原因就是因为文件系统不能给出完整性规则约束，这也是文件系统不能保证数据的正确性、有效性和一致性的本质原因。
在下一次呢,讨论一下数据模型中的概念模型。
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