了解关系数据库模型

The relational database model was a huge leap forward from the network database model.
关系数据库模型是网络数据库模型的一个巨大飞跃。
Instead of relying on a parent-child or owner-member relationship, the relational model allows any file to be related to any other by means of a common field.
关系模型不依赖于父子关系或所有者-成员关系，而是允许任何文件通过公共字段与任何其他文件相关联。
Suddenly, the complexity of the design was greatly reduced because changes could be made to the database schema without affecting the system's ability to access data.
突然，设计的复杂性大大降低，因为可以在不影响系统访问数据能力的情况下对数据库模式进行更改。
And because access was not by means of paths to and from files, but from a direct relationship between files, new relations between these files could easily be added.
并且由于访问不是通过文件之间的路径，而是通过文件之间的直接关系，因此可以轻松地添加这些文件之间的新关系。
In 1970, when E.F. Codd developed the model, it was thought to be impractical.
1970年，当e.f.Codd开发了这个模型，被认为是不切实际的
The increased ease of use comes at a large performance penalty, and the hardware in those days was not able to implement the model.
增加的易用性带来了很大的性能损失，并且当时的硬件无法实现该模型。
Since then, of course, hardware has taken huge strides to where today, even the simplest computers can run sophisticated relational database management systems.
当然，从那以后，硬件已经取得了巨大的进步，即使是最简单的计算机也可以运行复杂的关系数据库管理系统。
Relational databases go hand-in-hand with the development of SQL.
关系数据库与SQL的开发齐头并进。
The simplicity of SQL - where even a novice can learn to perform basic queries in a short period of time - is a large part of the reason for the popularity of the relational model.
SQL的简单性 -- 即使是新手也可以在短时间内学习执行基本查询 -- 是关系模型流行的很大一部分原因。

The two tables below relate to each other through the product_code field. Any two tables can relate to each other simply by creating a field they have in common.
下表通过product_code字段相互关联。任何两个表都可以通过创建它们共有的字段来相互关联。
Table 1

Product_code	Description	Price
A416	Nails, box	$0.14
C923	Drawing pins, box	$0.08

Table 2

Invoice_code	Invoice_line	Product_code	Quantity
3804	1	A416	10
3804	2	C923	15