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Database is an effective technology of data management , It is an important branch of computer development . Four concepts to understand before learning about databases ：

data ： The basic object stored in a database , It's a symbolic record of things .
database ： A warehouse for storing data , An organized and sharable collection of data stored in a computer for a long time .
Database management system ： A layer of data management software located between the user and the operating system , Scientific organization and storage , Efficient acquisition and maintenance .
Database system ： database , Database management system , Applications and database administrators .

1.2 The emergence and development of data management technology

Data management is generated according to the needs of data management tasks , Data management refers to the classification of data , organization , code , Storage , Retrieval and maintenance . Data management technology has experienced manual management , file system , There are three stages of database system .

Manual management ：
Data is not saved
Application management data
Data is not shared
Data is not independent

file system ：
Data can be saved for a long time
The file system manages the data
Poor data sharing , It's redundant
Data independence is poor

Database system ：
In order to meet the needs of multi-user and multi application data sharing , Make data serve as many applications as possible , The application of database system , Software that specifically manages data has emerged —— Database management system . It is suitable for large-scale data management , High sharing , Little redundancy , Overall structure , Safe and reliable .

1.3 Characteristics of database system

The database system provides powerful operation , For example, the query operation can be implemented with only one statement , Compared with file system, database system has the following characteristics ：

Data structure ： Realize the overall data structure , Not for an application , A whole oriented organization or enterprise , There is a connection between the data .
High data sharing , Low redundancy and easy expansion ： Data sharing reduces data redundancy , Save memory space , At the same time, it avoids the incompatibility and inconsistency between data .
High data independence :(1). Physical independence ： The physical storage of data in the user's application and database is independent .(2). Logical independence ： The logical structure of the user's application and database is independent .
The data is uniformly managed and controlled by the data management system :
Data security protection ： Protect data and prevent data leakage caused by illegal use .
Data integrity ： The correctness of the data , Effectiveness and compatibility .
concurrency control ： Control and coordinate when multiple users access and modify the database at the same time .
Database recovery technology ： When the database fails due to various reasons, the correctness of the data will not be affected .

2. Data model

A map , A model aircraft is a model , Models make people think of things in life . Data model is also a model , It's an abstraction of real world data features . Data model is used to describe data , Organizing and operating data .

2.1 Two types of data models

The data model should meet three requirements ： Real simulations show the world , It is understood by people and easy to realize by computer .

The first category ： conceptual model
The second category ： Logical model and physical model

1. conceptual model

Conceptual model is an intermediate level from the real world to the machine world , It's the first level of abstraction from the real world to the information world , It is a useful tool for database designers to design databases . The main concepts involved in the information world are ：

Entity ： Things that exist objectively and can be distinguished from each other
attribute ： The characteristics of an entity
code ： The attribute that uniquely identifies the entity
Solid type ： Use entity name and entity attribute set to abstract similar entities
Entity set ： A collection of entities of the same type
contact ： The relation between entities usually refers to the relation between sets of entities , There are one-to-one relationships between entities , One to many , Many to many

2. Entity - How to contact

conceptual model Modeling the information world , So the concept model can accurately express the common concepts of the information world . We use it E-R Figure to represent the conceptual model of the real world .

2.2 The components of the data model

The data model is usually composed of data structure , Data manipulation And data Integrity constraints Three parts .

data structure ： Description of the static characteristics of the system
Data manipulation ： A collection of operations allowed by objects in a database
Data integrity constraints ： The constraints and dependency rules of data and its connections

2.3 Common data models

The main logical data models in the database are ：
hierarchical model ：

Use tree structure to represent various entities and the relationship between entities . The structure is clear , High query efficiency , Provides good integrity support , But because there are many connections in the real world, they are non hierarchical , And the query child node must pass through the parent node , So there are drawbacks .
Mesh model

Mesh models are more common than hierarchical models , Multiple nodes without parent nodes are allowed , Multiple parent nodes are allowed . The mesh model can describe the real world better than the hierarchical model , And high access efficiency , however , It has some disadvantages such as complex structure .
relational model
Relationship name ： Table name
Relationship ： A relationship corresponds to a table
Tuples ： A row in the table is a tuple
attribute ： A column in the table is an attribute
code ： A property group in the table , A tuple can be uniquely determined
Domain ： A set of values with the same data type
component ： An attribute value in a tuple
Relationship model ： Relationship name ( attribute 1, attribute 2...)

Relational database The relationship model is adopted . A relational model consists of a set of relationships , The data structure of each relationship is a normalized two-dimensional table . The structure is clear , Access path is transparent to users , And it has higher data independence , Security . however , The query efficiency is often not as good as the previous formatting model , So this is the query optimization .
Object oriented data model
Object relational data model, etc

3. The structure of the database system

Database system usually adopts three-level mode structure .

3.1 The composition of database system pattern

There is... In the data model type and value The concept of , But a schema is a description of the logical structure and characteristics of all data in a database , Only type but not value . The pattern is relatively stable , But instances are variable .

3.2 Three level schema structure of database schema

Pattern ： Logical mode , It is a description of the logical structure and characteristics of all the data in the data .
External mode ： User mode , A description of the logical structure and characteristics of local data that database users can see and use .
Internal mode ： A database has only one internal schema , It is a description of physical structure and storage mode .

3.3 Secondary image of database

The three-level schema in a database is the three levels of abstraction of the database , It leaves the specific organization of data to the database management system . In order to realize the connection and transformation of three abstract levels within the system , The database management system provides two layers of images between these three-level patterns ： External mode / Pattern image and Pattern / Internal mode image .

4. Composition of database system

The database system consists of database , Database management system , Applications and Database administrator form .

5.MySQL Installation and configuration tutorial

Download and install the configuration MySQL Database management system encountered many problems , A detailed tutorial is attached , For reference .

1. download