E-R graph to relational model of the 2022 database of tyut Taiyuan University of Technology

Question 1

This kind of question is relatively simple ; Let's first solve the meanings of nouns that may appear in the problem , Then look at some examples and learn it

One 、

1、E-R Figure composition ：（1） Rectangle —— Entity （2） The ellipse —— attribute （3） The diamond —— Relationship

Two 、

1、 Primary key ： A primary key is an attribute or attribute group that uniquely identifies a row in a table （ This thing is easy to find , No teaching ）

2、 The difference between primary key and primary code ：（ Generally, no distinction is made , As a thing ）

A table can only have one primary key , But there can be multiple candidate indexes , When there are multiple candidate codes , You can select one as the main code , The selected candidate code is called primary key , And the main code can have multiple

3、 Foreign keys ： The foreign key of one table can be associated with another table , So as to define one to many relationship or many to many relationship

4、 An attribute or a group of attributes can be both a primary key and a foreign key

5、 Relationship model ： Short for R（A1,A2,~~~An）,R Name the relationship ,A For the property name

3、 ... and 、 Method （ First write the rectangle , Then write diamond , After writing, confirm whether the difficulties in one-on-one appear , The primary key is written in front ）

E-R How to transform a model into a relational model , There are three situations to explain , They are one-on-one , One to many and many to many .（ Because there are two ways , So often the answer to this question is not unique , I will only introduce the simplest ！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！！）

1. One on one situation ：

（1） Put all entities （ Rectangle ） And its corresponding properties （ An ellipse pointing to an entity ） In off mode                                        R （A1,A2,~~~An） In the form of ;

（2） Then put the relationship （ The diamond ） And its corresponding entity's main code and diamond's own attributes are in relation mode R（A1,A2,~~~An） The shape of               The formula is listed in turn , Among them, the relationship corresponds to R, The main code corresponds to A, Any master code can be used as the primary key of this relationship pattern

 （3） difficulty ： Put this difficulty in the example , Very important , Is the key to the full score of this question

2. One to many situation ：

（1） Put all entities （ Rectangle ） And its corresponding properties （ An ellipse pointing to an entity ） In relational mode                                      R （A1,A2,~~~An） In the form of ;（ Same as one-on-one ）

（2） Then put the relationship （ The diamond ） And its corresponding entity's main code and diamond's own attributes are in relation mode R（A1,A2,~~~An） The shape of               The formula is listed in turn , Among them, the relationship corresponds to R, The main code corresponds to A, The primary key of this relational schema must specify n The main code of the end is the relational module             The primary key of formula

3. Many to many situation ：

（1） Put all entities （ Rectangle ） And its corresponding properties （ An ellipse pointing to an entity ） In relational mode                                      R （A1,A2,~~~An） In the form of ;（ Same as one-on-one ）

（2） Then put the relationship （ The diamond ） And its corresponding entity's main code and diamond's own attributes are in relation mode R（A1,A2,~~~An） The shape of               The formula is listed in turn , Among them, the relationship corresponds to R, The main code corresponds to A, The primary key of this relational schema must be n End 、m End ... The main code of is               Same composition

Example ：（ The real question ）

  Explain ：