6 properties of database relationships

6 properties of database relationships

(1) Columns are homogeneous, that is, the components in each column are the same type of data , from the same domain.

(2) Different columns can come from the same domain, and each column is called an attribute. Different attributes should be given different attribute names.

(3) The order of the columns does not matter, that is, the order of the columns can be exchanged arbitrarily.

(4) Any two tuples cannot be exactly the same.

(5) The order of the lines does not matter, that is, the order of the lines can be exchanged arbitrarily.

(6) The components must take atomic values, that is, each component must be an indivisible database.

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Basic concepts of database systems

Data: It is actually a symbolic record that describes things.

Characteristics of data: It has a certain structure and is divided into types and values, such as integer type, real type, character type, etc. The value of the data gives a value that conforms to the stereotype, such as the integer value 15.

Database: It is a collection of data that has a unified structure and is stored in a unified storage medium. It is an integration of multiple application data and can be shared by various applications.

Data stored in the database is stored according to the data model provided by the data, and has the characteristics of integration and sharing.

Database management system: A system software responsible for data organization, data manipulation, data maintenance, control and protection, and data services in the database. It is the core of the database.

Database management system functions:

(1) Data schema definition: that is, constructing its data frame for the database;

(2) Physical construction of data access: for data The physical access and construction of the schema provide effective access methods and means;

(3) Data manipulation: Provide convenience for users to use database data, such as query, insertion, modification, deletion, etc. and simple arithmetic Operations and statistics;

(4) Data integrity and safety definition and inspection;

(5) Database concurrency control and fault recovery;

(6) Data services: such as copy, transfer, reorganization, performance monitoring, analysis, etc.

In order to complete the above six functions, the database management system provides the following data language:

(1) Data definition language: responsible for the definition of data schema and the construction of physical access to data;

(2) Data manipulation language: Responsible for data manipulation, such as query, addition, deletion, modification, etc.;

(3) Data control language: Responsible for the definition and inspection of data integrity and security As well as concurrency control, fault recovery, etc.

Data language has two structural forms according to its usage: interactive command (also known as self-contained or autonomous language) host language (generally can be embedded in some host language).

Database administrator: a professional manager who plans, designs, maintains, monitors, etc. the database.

Database system: a running entity composed of five parts: database (data), database management system (software), database administrator (personnel), hardware platform (hardware), and software platform (software).

Database application system: It consists of database system, application software and application interface.

File system stage: Provides simple data sharing and data management capabilities, but it cannot provide complete, unified, management and data sharing capabilities.

Hierarchical database and network database system stage: Provides strong support for unifying and sharing data.

Relational database system stage

The basic characteristics of the database system: data integration, high data sharing and low redundancy, data independence (physical independence and logical independence) , unified data management and control.

Three-level model of database system:

(1) Concept model: description of the global data logical structure in the database system, public data view for all users;

(2) External mode: also called submode and user mode. It is the user's data view, which is the data model seen by the user;

(3) Internal model: also called the physical model, it gives the physical storage structure and physical access method of the database.

Two-level mapping of database system:

(1) Mapping from conceptual schema to internal schema;

(2) Mapping from external schema to conceptual schema.

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