Relational databases and databases

Relational databases and databases

1. Relational databases

Relational Database: a database built on a relational model (mathematical model.

Relational Model: A so-called relational model. The relational model includes three aspects:

Data: data storage issues. Two-dimensional tables (with rows and columns)

Operation Command set: All SQL statements

Integrity constraints: Data constraints (fields and fields) in the table, and constraints between the table and the table (Foreign keys)

2. Relational Database Design

Relational Database: from the analysis of data needs to be stored, if a type of data (entity) should be designed as a two-dimensional table; the table is composed of the header (field name: used to specify the data name) and data components (actually stored data units)

Tables: rows and columns

Header	Field name 1	Field name 2
Data Unit	Data 1	Data Line

Analysis based on actual cases: Analysis of a teaching system, the lecturer is responsible for teaching, teaching students, teaching students in the classroom

① Identify the entities in the system: the lecturer table, student table, and class table

② Identify the data information that should exist in the object

INSTRUCTOR: name, gender, age, salary ......

Student: name, gender, student ID, subject ......

Class performance: Class Name, classroom number ......

Relational Database: maintains the relationships between entities and entities.

Internal entity contact: Each student has a name, gender, and subject information.

Name	Gender	Student ID	Subject	Age
Chen	Female	030310303	Chemical Engineering	22
Duhong	Male	030320305	Chemical engineering principles
Fu Wei		030330303	Physical Chemistry

All fields in the second row are describing the student Chen Ming (internal contact). The second column can only contain gender (internal constraint)-Internal Contact of the entity.

One of the characteristics of relational databases: If a field corresponding to a table does not have a value (data), but the system still needs to allocate space: relational databases are a waste of space.

The relationship between entities: Each student must belong to a certain class, and each class must have multiple students (one to multiple)

All fields in the second row are describing the student Chen Ming (internal contact). The second column can only contain gender (internal constraint)-Internal Contact of the entity.

One of the characteristics of relational databases: If a field corresponding to a table does not have a value (data), but the system still needs to allocate space: relational databases are a waste of space.

The relationship between entities: Each student must belong to a certain class, and each class must have multiple students (one to multiple)

Name	Gender	Student ID	Subject	Age
Chen	Female	030310303	Chemical Engineering	22
Duhong	Male	030320305	Chemical engineering principles
Fu Wei		030330303	Physical Chemistry

Class table

Class Name	Classroom no.
Chemical engineering principles	B203
Physical Chemistry	A305

Q: Can the two tables determine which student belongs to which class or which class has which students?

Solution: Add a class field to the student table to point to the class (a unique class information must be found)

Name	Gender	Student ID	Subject	Age	Class Information
Chen	Female	030310303	Chemical Engineering	22	Chemical engineering principles
Duhong	Male	030320305	Chemical engineering principles		Physical Chemistry
Fu Wei		030330303	Physical Chemistry		Physical Chemistry

Association between student entities and class entities-relationships between entities