Database System Principle-ER model and relational model, database system principle er

Database System Principle-ER model and relational model, database system principle er

The database system is a type of software, and the database system naturally has its own life cycle-lifetime. Its survival period starts from planning to uninstalling it. The intermediate process is very complicated. In order to realize users' ideas, database personnel abstract the real-life data and then classify and operate the data to integrate the data, they are encapsulated into a system for users to use. Finally, the database personnel are constantly maintaining their results, which not only meets the needs of users, it also embodies the idea of serving the people wholeheartedly.

1. Relationship between software and database

A database system is a subclass of a super-class software instance. Of course, you can also regard the software as a strong entity, which is a weak entity in the database system, A weak entity depends on a strong entity. In the database design stage, the transformation of the conceptual model to the relational model is a key point. In the conversion process, data must be matched one by one. Of course, data redundancy must also be eliminated. During database design, database designers need to abstract the data in the real world and the relationships between the data, which are represented by entities, links, and attributes. It is the most time-consuming and troublesome step for database maintenance. After the database is built, it will continue to grow old along with the database.

2 ER Model

The ER Model consists of entities and links. Entities are abstract in real life, and information transmission between entities is achieved through links. An attribute is a business card shared by entities and contacts. Attributes include simple and composite attributes, single-value and multi-value attributes, storage attributes, and derived attributes. In composite attributes, each attribute has a certain logical dependency, or hierarchical relationship, such as the book address attribute, which can be divided into provinces, cities, counties ...... The confrontation attribute does not have a certain relationship between them. For example, a degree can include a master's degree, a doctor, a bachelor's degree, and so on.

Operations on the ER model are actually operations on the ER model elements, that is, operations on entities, connections, and properties, the purpose of the operation is to achieve user requirements while reducing data redundancy.

2 conversion from ER model to Relational Model

The transformation from the ER model to the relational model is to further organize the abstract data. After the conversion is complete, the data can be displayed in two-dimensional tables, which facilitates the data stored in the database. Database Operations on data.

The ER model is a tool for people to understand the objective world. The relational model is the product of converting the ER model into the DBMS world. They are designed to continuously meet the needs of users.

Database System principles relational data models are converted into ER Models

Based on the relational database model, the ER diagram contains three entities and one link:
Entity 1 --- a driver, whose attributes are the driver's license number, name, address, phone number, and zip code. The driver's license number is the main code.
Entity 2 --- vehicle, attribute: vehicle license plate number, model, production date, manufacturing plant, where vehicle license plate number is the main code
Entity 3 --- penalty method, attribute: penalty number, warning, penalty, temporarily detained driving license, where penalty number is the main code
Contact 1 --- violation. The unique property violation number is the primary code, which is connected to the preceding three entities in multiple ways.

ER model: Model A, Conceptual Model B, hierarchical model C Network Model D, relational model

A conceptual model, followed by A Class