Converting SQL to Relational Algebra / Calculus

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postgresql relational-theory relational-algebra relational-calculus

                          I have designed a schema and generated a few SQL queries. I'm using PostgreSQL. 
 
For example:

    CREATE TABLE train 
    (
      train_code SERIAL PRIMARY KEY,
      name TEXT NOT NULL
    ); 

    CREATE TABLE journey 
    (
      journey_id SERIAL PRIMARY KEY, 
      int INTEGER,
      train_code REFERENCES train(train_code)
    ); 

    CREATE TABLE price 
    (
      journey_id REFERENCES journey(journey_id),
      price INTEGER
    ); 

Give me all train names that commence from train_code NYC or SFO that are priced $50 or more. (assume the price table is in $ already). 

    SELECT train.name
    JOIN   train
    JOIN   journey on train.train_code = journey.train_code
    JOIN   price on price.journey_id = journey.journey_id 
    WHERE  price.price >= 50 
    AND    journey.train_code IN ('NYC', 'SFO') 
    AND    journey.int = 1 -- (first train of the journey)
 
I am unsure how to convert this into a relational algebra and/or calculus query. I have looked at a few converters e.g. http://dbis-uibk.github.io/relax/calc.htm  but in this calculator 'join' for example and 'in' is not allowed. 




                        

Asked by Dino Abraham (127 rep)

Jun 20, 2017, 02:48 PM
Last activity: Mar 22, 2021, 02:17 PM

Converting SQL to Relational Algebra / Calculus

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