mysql - 在关系代数/微积分中表达 SQL 查询

Question

如何在关系代数/微积分中表达 SQL 查询

SQL

SELECT  * 
  FROM Product a 
  LEFT JOIN Offer b ON a.ProductID = b.ProductID
 WHERE b.ProductID IS NULL

SELECT  * 
  FROM Dealer a 
  JOIN Offer b ON a.DealerID = b.DealerID
  JOIN Product p ON b.ProductID = p.ProductID
 WHERE p.ProductName = Armaniwear

Answer 1

你看过这个吗: Relational Algebra Calculator ？

实际上，您只需在那里创建“关系”(基于您的表)，它将有助于完成其余的事情。

因此，例如，您首先转到组编辑器并按如下方式创建关系:

group:MyGroup

Products = {
    ProductId:number, ProductName:string
}

Dealer = {
    DealerId:number, DealerName:string
}


Offer = {
    OfferId:number, OfferName:string, DealerId:number, ProductId:number
}

现在，转到SQL选项卡并输入您的查询:

SELECT * FROM Dealer
         INNER JOIN Offer 
                 ON (Dealer.DealerId = Offer.DealerId)
         INNER JOIN Products
                 ON (Offer.ProductId = Products.ProductId)  
   WHERE Products.ProductName = 'Armaniwear';

编辑器似乎不接受查询中表的别名。但是，我相信当您获得最终结果时，这种权衡并没有那么糟糕:

σ Products.ProductName = 'Armaniwear' Dealer
⨝ ( Dealer.DealerId = Offer.DealerId ) Offer 
⨝ ( Offer.ProductId = Products.ProductId ) Products