Er and eer models

The idea of this paper mainly comes from Luo Hao jackfrued The teacher's online class
For my study reference only , Not used for other purposes ！

Readers are also advised to learn through the teacher's course .

Introduce

Why would there be ER Map and EER Existence of graph ？

** Really do the project 、 When designing a database ,** It's not that simple , For example, many users can't see 、 But for convenience DBA Fields created using . Such as id, There are usually two Date Field （ A indicates when the record was created , Two indicates the last update time of this record ）, And reserve a VARCHAR / json Field ; There are other considerations （ such as auto-increment Constraints are not commonly used in development , Use more algorithms, such as distributed ID generating algorithm （ Such as SnowFlake……） Of course, this is not important for the course ） It's impossible to write as soon as you come up SQL Of the statement , You need to design the structure of the table and the relationship between the tables first .

ER Model

ER chart （Entity Relationship, Entity relations ） So there comes . Here is a ER An example of a graph , The rectangular box represents ： surface （ That's entity ）, The oval box represents ： Fields in the table （ Properties of entities ）, The diamond box represents ： Relationship , The multiplicity of the relationship is indicated on the connecting line .

Compared with a large section of SQL Create table statement , As long as you can understand ER chart , The structure of the table 、 The relationship is clear at a glance .

EER Model

Comparison ER The model has more generalization levels 、 Aggregation hierarchy 、 Concepts such as weak entities .

Generalization hierarchy

Include generalization and specialization （ Generalization and specialization ）、 Parent class （superclass） And subclasses （subclass） The concept of .

Subclass parent class Just like that. Java Inheritance , If animals are the parent , cat 、 Dogs are a subclass .

generalization Also called induction , Is to extract the common attributes of several classes as the parent class ;

specialized Also known as deduction , Is to add their own special attributes as subclasses on the basis of the parent class .

among , Subclasses and superclasses have two relationship attributes ：mandatory and optional、disjoint and overlapping.

mandatory / optional： All attributes in the parent class must be contained in each subclass / Don't inherit all , Select partial inheritance （ Completeness limitation ）

disjoint / overlapping： Whether there can be intersecting properties between several subclasses inheriting the same parent class （ Intersection restrictions ）

In the figure The transportation tool is the parent class , The plane 、 train 、 Cars are a subclass . Write... In the circle D / O, Express disjoint / overlapping. There is a double solid line between the parent class and the circle , Said is mandatory Complete inheritance .

Aggregation hierarchy

Aggregation, The collection hierarchy is no longer different from the parent and child classes , But by the …… The difference in composition .

Pictured , room 、 doors and windows 、 The computer 、 The projector is an integral part of the classroom , It's not inheritance .

Weak entity

One entity is meaningful only when another entity exists . Such as parents and children , Without one side, the other side is meaningless .

Workbench And other tools support painting ER chart , Even after drawing, it can be automatically generated SQL Statement table building . Workbench The picture in is EER chart （ Extended ER chart ）

stay EER Below , Click on DATABASE - FORWARD ENGINEER Forward engineering , You can select the table to generate 、 Field , Generate SQL Statement creation table .

Automatically generated SQL In the sentence , There are two sentences below the foreign key ON DELETE NO ACTION ON UPDATE NO ACTION, Meaning for ： When the primary key of the foreign key reference is modified / deleted , How will the table where the foreign key is located be affected ？—— Take no action . It is suggested to remove these two sentences . If you remove , You can't just modify / Delete the primary key used by the foreign key .

similarly , stay SQL Statement page , Click on DATABASE - REVERSE ENGINEER Reverse Engineering , Can be generated according to the structure of the table EER chart .

Power Designer Modeling tools , Also support positive / Reverse Engineering , Can generate SQL dialect . Full version paid .