Paradigm in database: first paradigm, second paradigm, third paradigm

Tuples ： A line in the table

code ： A column in the table

Candidate code ： can Unique identification , Such as through （ Id card number ） perhaps （ School , class , Student number ） Can uniquely identify a person ; Main attribute ： Attributes that have appeared in the candidate code

Main code : Also called primary key , Choose a boss from the candidate code

Foreign code is also called foreign key . If An attribute in a relationship yes The main code in another relationship Then this attribute is foreign code

First normal form , Second normal form , Super simple introduction of the third paradigm

First normal form : Can not be further divided

For example, there are two attributes in a table ： company 、 goods （ name 、 Number ）, However, the commodity can be identified by its name 、 Quantity means , Do not conform to the , and company 、 name 、 Number accord with

Second normal form ： Based on the first paradigm , Eliminate partial dependence

The order number	Product number	Product quantity	Product discounts	product price	Order amount	The order time
10001	25	100	0.9	9	5000	2022-07-22
10001	26	200	0.8	9	7000	2022-07-23

$$\begin{gathered} booksingleNumber、productionproductNumber\to productionproductCountThe\; amount、productionproductfoldbuckle、productionproductpricegrid \\ booksingleNumber\to booksinglegoldforehead、booksinglewhenbetween \end{gathered}$$

‘ The order number ’ and ‘ Product number ’ Is the primary key of the table （ Main code ）, These two fields together determine ‘ Product quantity ‘ ’ Product discounts ‘ ’ product price ‘ ,

and ‘ Order amount ’ and ‘ The order time ’ Only by ‘ The order number ’ decision , Not by Primary key （‘ The order number ’ and ‘ Product number ’） Joint decision , therefore ‘ Order amount ’ and ‘ The order time ’ yes Depends in part on the primary key

Correct version

The order number	Product number	Product quantity	Product discounts	product price
10001	25	100	0.9	9
10001	26	200	0.8	9

The order number	Order amount	The order time
10001	5000	2022-07-22
10001	7000	2022-07-23

Third normal form ： Based on the second paradigm , Eliminate delivery dependency

ID number	full name	Gender	Age
34666666666	Zhang San	male	22

$$bodyShareProve\to surnamename\to sexother、yearage$$

“ Gender ” and “ Age ” Although indirect dependence ‘ ID number ’, but Directly dependent on Yes. “ full name ”, Correct version

ID number	full name
34666666666	Zhang San

full name	Gender	Age
Zhang San	male	22

The transfer function depends on ： $bodyShareProve\to surnamename\to sexother、yearagebutyessexother、yearage\nrightarrow surnamename\nrightarrow bodyShareProve$