如何处理只有两个可能值的属性

Question

在我的 Web 应用程序中，我有一个 company，它可以是 buyer 或 supplier 或两者兼而有之。
所以我的数据库表会是这样的

Company( id_company, ..., is_buyer, is_supplier, ... )

或者：

Company( id_company, ... )
Type_company( id_type_company, type )
Extra_table(id_company, id_type_company )

或者：

Company( id_company, ... )
Type_company( id_company, id_type_company, type )

如果可能的话，我希望对每种情况都有一个解释（优点和缺点）。

Answer 1

你可以考虑使用像这样的通用超类型

CREATE TABLE companies
(
  id int not null primary key, 
  name varchar(128)
  -- other columns
);
CREATE TABLE buyers
(
  company_id int not null primary key,
  foreign key (company_id) references companies (id)  
);
CREATE TABLE suppliers
(
  company_id int not null primary key,
  foreign key (company_id) references companies (id)
);

以下是一些示例查询：

-- Select all buyers
SELECT c.id, c.name
  FROM companies c JOIN buyers b 
    ON c.id = b.company_id;

-- Select all suppliers
SELECT c.id, c.name
  FROM companies c JOIN suppliers s 
    ON c.id = s.company_id;

-- Select companies that are both buers and suppliers
SELECT c.id, c.name
  FROM companies c JOIN buyers b 
    ON c.id = b.company_id JOIN suppliers s 
    ON c.id = s.company_id;

-- Select companies that are buers BUT NOT suppliers
SELECT c.id, c.name
  FROM companies c JOIN buyers b 
    ON c.id = b.company_id LEFT JOIN suppliers s 
    ON c.id = s.company_id
 WHERE s.company_id IS NULL; 

这里是SQLFiddle演示

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