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I have a question involving relational algebra. Suppose I have two relations A and B, where B has a foreign key that references A.

Using relational algebra, how do I select all tuples from A where variable X exists in B? Example:

Relation 1

Cars(car_id,model)    

Relation 2

Ford(car_id,year)

How do I select from Cars, all the tuples where id exists in Ford? Example:

Cars  car_id  model
      -------------
       1    a
       2    b
       3    c

Ford  car_id   year
      1        2000
      3        2002

What I would like is: select all from cars where id exists in Ford, which returns:

Result:  car_id  model
        --------------
         1    a
         3    c

closed as off-topic by ypercubeᵀᴹ, Philᵀᴹ, Kin Shah, Shanky, Colin 't Hart Oct 5 '15 at 13:41

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Too localized - this could be because your code has a typo, basic error, or is not relevant to most of our audience. Consider revising your question so that it appeals to a broader audience. As it stands, the question is unlikely to help other users (regarding typo questions, see this meta question for background)." – ypercubeᵀᴹ, Philᵀᴹ, Kin Shah, Shanky, Colin 't Hart
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4

The (somewhat trivial) answer seems to be:

Cars ⋉ Ford

Relational algebra, semijoin (Wikipedia)

Alternatively (in response to a comment):

πR(R ⋈ S)

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