database_2_relational database

Relational Database

1. What is a relational database?

Relational database: is a database built on a relational model (mathematical model).

Relational Model: A so-called model built on relationships. 

The relational model includes three aspects: 1. Data structure: the problem of data storage, two-dimensional table (with rows and columns) 2. Operation instruction set: all SQL statements 3. Integrity constraints: data constraints in the table (the relationship between fields and fields) between tables), constraints between tables (foreign keys)

2. Design of relational database?

Relational database: Analyze from the data requirements to be stored. If it is a type of data (entity), it should be designed as a two-dimensional table. The two-dimensional table is composed of a table header (field name: the name used to specify the data) and a data part ( actual stored data unit)

2D Table: Rows and Columns

header	field name 1	field name 2
data unit	data 1	data 2

Dealing with real cases: analyzing a teaching system, the lecturer is responsible for teaching, teaching students, teaching students in the classroom

1. Find out which entities exist in the system: Lecturer table, Student table, Class table
2. Find out the data information that should exist in the entity Lecturer: Name, Gender, Age, Salary Student: Name, Gender, Student ID, Subject Class: Class Name, Classroom ID

Relational database: maintain the relationship between entities and between entities

Inter-entity links: each student has name, gender, student ID, subject information

Student table:

Name	gender	student ID	Subject	age
Chen Ming	male	1001	PHP	20
Chen Jingjing	Female	1002	PHP
Wang Wei		1003	UI

All the fields in the second row describe the student (internal connection), and the second column can only put gender (internal constraints)

One of the characteristics of relational databases : if there is no value (data) in a corresponding field in the table, but the system still needs to allocate space, relational databases waste space.

Entity to entity connection : each student must belong to a certain class, and each class must have multiple students (one-to-many)

Class Schedule:

class name	classroom number
PHP0810	B205
UI0810	A203

Solution to the association problem between two tables: add a class field to the student table to implement the class (must be able to uniquely find a class information)

Name	gender	student ID	Subject	age	class name
Chen Ming	male	1001	PHP	20	PHP0810
Chen Jingjing	Female	1002	PHP		PHP0810
Wang Wei		1003	UI		UI0810

Association relationship between student entity and class entity: relationship between entity and entity