功能依赖（第一步）

Question

我正在练习Functional Dependencies，我正在看一本与数据库相关的书，他们提供了一个例子，它是这样的：

A company performs all kinds of sales through different terminals (terminals for payment of debit and credit cards). These terminals belong to different sectors, from hypermarkets to simple candy kiosks. We are interested in knowing the business name, a contact telephone number and legal address, including City and State which are tabulated. The terminals have an identifier that distinguishes them and are sold to a single store (not reuse between different shops). The terminals also have an operating system. Each of the terminals have a certain amount of memory (in MBytes), manufacturing country (also tabulated) and year of importation. Terminals have a ticket reprint service, so they need to store the unique number of the terminal that made the sale, the invoice number associated with it, the date and time of the sale, the amount, the type of card (debit or credit), the card number and in how many installments the payment was made.

很明显，他们给了我一段感情；我必须努力获得功能依赖性，你明白我的意思吗？

这是关系：

Company(BusinessName, FantasyName, BusinessNumber, Telephonenumber, address, locality, locality_number, state, state_number, terminalid, operatingsystem, idCountry, Country, ImportationYear, idTicket, Billnumber, amount, CardNumber, Memory_quantity, date_and_time_of_sale, card_type, installments_quantity)

所以，首先我替换了那个关系的值；我用字母：

Number of elements inside Company: 22

那么：公司(A, B, C, D, E, F, G , H, I, J , K, L M, N , O, P ,Q, R, S ,T, U, V )

现在，我必须找到依赖项，例如：

A -> B, C

E -> L, M, R

T -> U, V

不太确定，可以帮我定义函数依赖关系吗？。我很困惑：/

编辑：

BusinessName 
FantasyName 
BusinessNumber 
Telephonenumber 
address 
locality 
locality_number 
state 
state_number 
terminalid 
operatingsystem 
idCountry 
Country 
ImportationYear 
idTicket 
Billnumber 
amount 
CardNumber
Memory_quantity 
date_and_time_of_sale
card_type 
installments_quantity

编辑#2：

函数依赖：

BusinessNumber -> BusinessName, FantasyName, TelephoneNumber, Address, LocalityNumber, StateNumber, IdCountry

Terminal -> operatingsystem, idCountry, ImportationYear, Memory_quantity

Sales -> amount, CardNumber, date_and_time_of_sale, card_type, installments_quantity

Answer 1

第 1 步 - 识别属性“组”：

根据您提供的属性的平面列表，可以看出这些属性可以按逻辑/“真实世界”/“业务域”所有者分组。例如，这些属性由现实世界的业务实体“拥有”：

BusinessName             Business
FantasyName              
BusinessNumber           Business
Telephonenumber          Business
address                  Business
locality                 Business (This attribute is redundant as locality_number serves the same purpose)
locality_number          Business
state                    Business
state_number             Business (This attribute is redundant as state_number serves the same purpose)
terminalid 
operatingsystem 
idCountry 
Country 
ImportationYear 
idTicket 
Billnumber 
amount 
CardNumber
Memory_quantity 
date_and_time_of_sale
card_type 
installments_quantity

这些由支付终端实体“拥有”：

BusinessName
FantasyName              
BusinessNumber
Telephonenumber
address
locality
locality_number
state
state_number
terminalid                 Terminal
operatingsystem            Terminal
idCountry                  Terminal
Country                    Terminal (Should be renamed to "ManufacturingCountry")
ImportationYear            Terminal
idTicket 
Billnumber 
amount 
CardNumber
Memory_quantity 
date_and_time_of_sale
card_type 
installments_quantity

当我浏览列表时，我想到了这些组：

BusinessName             Business
FantasyName              Business
BusinessNumber           Business (I assume this is a unique identifier, a "natural key")
Telephonenumber          Business
address                  Business
locality                 Business (This attribute is redundant as locality_number serves the same purpose)
locality_number          Business
state                    Business
state_number             Business (This attribute is redundant as state_number serves the same purpose)
terminalid               Terminal
operatingsystem          Terminal
idCountry                Terminal, Business 
Country                  Terminal, Business (This attribute is redundant as idCountry serves the same purpose)
ImportationYear          Terminal
idTicket                 Sale
Billnumber               Sale (What is the difference between `idTicket` and `BillNumber`?)
amount                   Sale
CardNumber               Sale -- NEVER STORE CREDIT CARD NUMBERS IN A DATABASE!!!!!
Memory_quantity          Terminal
date_and_time_of_sale    Sale
card_type                Sale
installments_quantity    Sale

第 2 步 - 转换为 tables（“关系”）并识别哪些属性是键（自然键或合成键），哪些只是普通数据（依赖数据属性）

CREATE TABLE Businesses (
    -- Natural key:
    BusinessNumber,

    -- Plain ol' data attributes (i.e. dependent attributes):
    BusinessName,
    FantasyName,
    TelephoneNumber,

    -- Business address fields (also plain ol' data):
    address
    locality_number,
    state_number,
    idCountry
)

CREATE TABLE Terminals (
    -- Natural key:
    terminalid,

    -- Plain ol' data attributes:
    operatingsystem,
    idCountry,
    ImportationYear,
    Memory_quantity
)

CREATE TABLE Sales (
    -- Natural key:
    idTicket,
    BillNumber,
    
    -- Plain ol' data attributes:
    amount,
    CardNumber,
    date_and_time_of_sale,
    card_type,
    installments_quantity
)

我还需要为推断的实体添加tables，例如Country和Locality（在现代应用程序开发中我们通常不引用states/countries 通过合成键（又名“代理键”），因为它增加了复杂性而没有增加任何业务价值）。

CREATE TABLE Localities (
    -- Synthetic key:
    locality_number,

    -- Plain ol' data attributes:
    Locality
)

CREATE TABLE Countries (
    -- Synthetic key:
    idCountry,

    -- Plain ol' data attributes:
    oCountry
)

第 3 步 - 添加外键：

Sales table 需要修改，以便它可以引用其相关的 Business 和 Terminal 记录（这应该在第 1 步中通过确定terminalId 和 BusinessNumber 是 Business、Terminal 和 Sale 的属性，顺便说一句——那是我的错误）：

CREATE TABLE Sales (
    -- Natural key:
    idTicket,
    BillNumber,
    
    -- Foreign keys:
    CompanyNumber,
    TerminalId

    -- Plain ol' data attributes:
    amount,
    CardNumber,
    date_and_time_of_sale,
    card_type,
    installments_quantity

    CONSTRAINT PK_Sales PRIMARY KEY ( idTicket, BillNumber )
    CONSTRAINT FK_Sales_to_Businesses FOREIGN KEY ( CompanyNumber ) REFERENCES Businesses ( BusinessNumber )
    CONSTRAINT FK_Sales_to_Terminals FOREIGN KEY ( TerminalId ) REFERENCES Terminals ( TerminalId )
)

（并在 Businesses 和 Terminals table 中添加 FOREIGN KEY 约束以引用 Localities 和 Countries）

功能依赖（第一步）

Functional dependencies (first steps)

sql

functional-dependencies

第 1 步 - 识别属性“组”：

第 2 步 - 转换为 tables（“关系”）并识别哪些属性是键（自然键或合成键），哪些只是普通数据（依赖数据属性）

第 3 步 - 添加外键：