Expressing SQL Queries in Relational Algebra/Calculus

Question

How can I express a sql query in a relational algebra/calculus

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

Have you had a look at this: Relational Algebra Calculator?

Effectively, you just have to create the "relations" (based on your tables) there and it will help with the rest of the things.

So, for instance you first go to Group Editor and create your relations as follows:

group:MyGroup

Products = {
    ProductId:number, ProductName:string
}

Dealer = {
    DealerId:number, DealerName:string
}


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

Now, go to the SQL tab and input your query:

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

The editor doesn't seem to accept aliases for the tables in the query. However, I believe the trade-off isn't as bad when you get the final result as:

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