WEBVTT

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在这一期视频里，我们将学习关系模型。

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关系模型已有超过

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35年历史，

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是数据库管理系统的基础。

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数以十亿计的产业由此产生。

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目前所有

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商业数据库系统采用关系模型。

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事实上它非常简单，这也是它优点之一。

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此外，它是可检索的。

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这里指我们可以

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用高级语言

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查询数据库。

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高级语言是简单的，

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用于查询数据库时可读性极高。

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最后一点，非常重要的，

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是可以高效的实现

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关系模型

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及其查询语言。

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让我们继续

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了解关系模型的基础构建。

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首要的构建是关系。

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一个数据库是由

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一系列关联或者

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署名的"表"组成。

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在我们的例子中有两组关系。

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我们的例子是一个

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学院申请人的模拟数据库。

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现在我们分开看学生和学院。

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我们有两个表，

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命名为

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学生和学院。

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顺便提一点，

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数据库行业里有一个

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关于表名应该是单数还是负数的

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有益的争论。

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我个人觉得无所谓。

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我就用单数吧。

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接下来，我们要讲属性。

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每一个关系和

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关系数据库都有一个事先

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命名的列或者属性。

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在学生表单里，

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比如说每一个学生

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都有一个ID,

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名字，GPA 和照片。

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在学院表单里，

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比如说

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每一个学院都有

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一个名字，州，招生人数。

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我们简写成 ENR。

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这些都是标好的列。

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现在的数据

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被存储在

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表单的元组（或者行）里。

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让我们在表里

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录入一些数据组。

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让我们从

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学生开始，比如说

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第一个学生的ID是123。

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名字是Amy，

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GPA 3.9， 她对此很满意。

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所以有一张微笑的相片。

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我们的第二个学生是234，

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Bob， 他的GPA

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是 3.4 .

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对此他并不开心。

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通常，

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一张表有上千，

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或者上百万，甚至上亿的

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行，每一行的每一个属性都有对应的值。

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在学院表单中，我们假设

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当然

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我们从Stanford开始

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California，招生人数是15000.

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