Database System Principle-ER model and relational model

I still remember the feeling of reading tianshu when I read database system principles for the first time, and the cloud was in the fog. Now I have read this book for the second time, and I suddenly felt something, "Oh, it turned out to be like this !" Now, let's get started. Write it down and take it as a note.

ER Model

The basic elements of the ER model are: entities, contacts, and attributes.

• Entity: a data object refers to an objective object that can be differentiated in an application. (Entities in the ER model often refer to entity sets)

Entity set: A set composed of the same class of entities.

The object is represented by a box, and the name of the object is indicated in the box.

• Link: the relationship between one or more entities.

The contact is represented in a diamond frame and linked to the relevant entity using a line segment.

• Attribute: a feature of an object is called an attribute.

The attribute is represented in an elliptical box, And the underlined attribute is the identifier.

Example: A potential design ERTU of Teaching Management

 

LINK model

Relational Model: represents the data model of the association between entities in the form of a two-dimensional table.

• In a relational model, a field is called an attribute, a field value is called a property value, and a record type is called a relational model.
• The link mode name is R. A record is called a tuple, and a set of tuples is called a link or an instance.
• Generally, uppercase letters A, B, C ,...... Represents a single attribute, and a lower-case letter represents the attribute value. The number of attributes in a link is called the "number of elements", and the number of tuples is called the "base ".
• It is also called a table, a row as a tuples, and a column as an attribute.

Instance 1: relational mode R

A B C D

A1 b1 c1 d1 A2 b2 c2 d2 A3 b3 c3 d3 A4 b4 c4 d4

 

Example 2: relational mode R (S #, C #, CNAME, TNAME). Its Attributes indicate the student ID, course number, course name, and instructor name of the elective course respectively.

 

S # C # CNAME TNAME

S2 C4 PASCAL WEN S4 C4 PASCAL WEN S6 C2 ADA LIU S3 C6 BASIC MA