Conceptual Model of Data Model

I learned database (4) -- after learning about the common characteristics of the data model, I would like to have a deep understanding of the concept model. Next time I will study the logic model, and the physical model will not be detailed, this involves the implementation of DBMS, which is too profound, so I am not capable enough in my personal circumstances. As mentioned above, the conceptual model is a modeling of the real world

I learned database (4) -- after learning about the common characteristics of the data model, I would like to have a deep understanding of the concept model. Next time I will study the logic model, and the physical model will not be detailed, this involves the implementation of DBMS, which is too profound, so I am not capable enough in my personal circumstances. As mentioned above, the conceptual model is a modeling of the real world

I learned database (4) -- Conceptual Model of Data Models

After learning about the commonalities of the data model, I will take a deeper look at the conceptual model. Next time I will study the logical model, I will not look into the physical model, because it involves the issue of DBMS implementation, which is too esoteric, my personal skills are insufficient.

As mentioned above, the conceptual model is a modeling of the real world and intended for users. Therefore, it must be able to fully express the semantics of applications and be easy to understand. A conceptual model is also called an information model. It is the first abstraction from the real world to the information world and requires support from some basic concepts. I have read some articles on databases before learning these concepts. These concepts are very important for my database study in the future. So I should study these concepts well, here, I will share my learning results.

Entities, objective things that do not shift by subjective will, can be different from each other. For example, people, things, and things can also be abstract concepts or links to this data. An employee, a student, a department, and a course are all entities.

Attributes, entity features, such as a person, name, and gender, are all attributes. An entity in a database is expressed by attributes.

The property of a unique identifier object is integrated into a code. This is actually a good understanding. For example, if a person identifies an object by ID card number, the ID card number is a code; sometimes the name cannot work, because there are people with the same name, when I call someone, if there is a person with the same name, I don't know who I am calling.

Domain, the value range of the attribute, virtual host, is the meaning of the value range, such as the Gender domain (male, female), but it is difficult to do this.

Real-size entities use entity names and attribute names to portray objects with the same attributes. This is a model, for example, a student (student ID, name, gender ,......).

An entity set is a set of the same type. For example, all students are an entity set.

In life, there is a link between various things and between things. The abstract to the information world is the relationship between the entity and the entity or between the entity and the entity. An internal link refers to the relationship between attributes. An external link refers to the relationship between entities in different entity sets. abstraction refers to the relationship between entities.

Here is a question:

(9400, Zhang Si, male, 19881219, communication system, 2007) indicates a student;

Students (student ID, name, gender, birthday, Major, and level) are real students;

{(9400, Zhang Si, male, 19881219, Xin Guan, Hong Kong virtual host, 2007), (9401, Zhang San, male, 19881211, Xin Guan, 2007 ),......} Represents an entity set.

Let's take a look at the relationships between entities, entities, and entity sets: entities are abstracted directly from life. Entities abstract the common attributes of entities, and entity sets are a set of entities, the entity is determined by the attribute value at a certain time point. It can be understood that the entity and entity are the same as the data type and variable, such as the int type variable I, and the value is 0.

Let's take a look at the relationship between real bodies. The relationship between real bodies is actually a certain business relationship between entity sets. One-to-one, one-to-many, and many-to-many. If each entity in entity set A is associated with N entities (N <= 1) in entity set B, and vice versa, the two entity sets have one-to-one relationship, it is recorded as. If entity set B has N entities (N> 1) associated with it, otherwise, each entity A in entity set B has at most one corresponding object, entity Set A has A one-to-multiple relationship with B, which is recorded as 1: N. If each entity in entity set A has N entities (N> 1) corresponding to B, on the contrary, each entity in US space, B, and A also have N entities (N> 1) corresponding to it, and instruct the two entity sets to have many-to-many connections. In fact, one-to-one and one-to-multiple connections are special cases. Therefore, the N value in the definition of multiple-to-multiple connections must be greater than one. I drew a picture using the Visio software, for example:

In addition, many materials mentioned the relationship between more than two entities. I think there are one hundred entities. I also turn them into any two links for analysis, therefore, I did not know more about the relationship between two or more entities.

Experienced netizens may have discovered that there is also a situation in which there is a single physical connection. This situation is no stranger. For example, the relationship between leaders and the leaders in the real-size employees is the case for different functional departments at the lower level under the department. Both of them are one-to-many connections, such:

After learning about the basic concepts of the conceptual model, let's take a look at how to represent this model. One representation method is the entity-contact method. This method is only available in part here, so we can learn more about the database design in the future. This method uses a E-R diagram to describe the concept model of the real world. It provides a method to represent the real shape, attributes and connections. The real shape is represented by a rectangle, the attributes are represented by an elliptic shape, and the links are represented by a diamond shape. The real shape and attributes are connected by an undirected edge. Other links also have attributes, they are also connected with undirected edges.

Now, this article is written, and the logic model will be learned next time. By the way, you should pay more attention to your health during the learning process. Here, I will post a [old Chinese medicine practitioners] on the Internet to share with you: Sweet Potato has many advantages, and shiitake mushrooms have been used as anti-cancer drugs; cabbage laxity detoxification, melon beans swelling and diuretic; tomato nourishing blood with skin, celery blood pressure anti-aging; bitter gourd clear heart and clear eyes, cucumber weight loss and clearing heat; corn inhibiting cholesterol, hawthorn blood pressure anti-aging; peanut lipid-Lowering Treatment of anemia, Health brain wufa Eat walnut; sesame oil and black, honey ishou and rundry; choose diet based on the situation, diverse eating good nutrition.

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