Selecting from three Relation with Relational Algebra

问题

I have for ex this Tables

Customer{cid, name, phone, address}
Employee{eid, name, positon, salary}
Resp_for{cid,eid}

Now I want to ask in relational Algebra that it should print out all employees that are resposible for the Customer: 'Obama'

I am trying it but not sure that I understand the relational algebra clearly, so I create this:

PROJECT   (SELECT                 (Customer x Employeee x Resp_for))
       E.eid       C.name = 'Obama'
                 AND R.cid = C.cid
                 And R.eid = E.eid

so how it looks as sql query?

SELECT E.eid
FROM Customer JOIN Employee JOIN Resp_for
WHERE C.name = 'Obama'
      AND R.cid = C.cid
      And R.eid = E.eid

is that correct?

回答1:

The correct SQL query for This wil be:-

SELECT E.EID FROM CUSTOMERS C, EMPLOYEE E, RESP_FOR R
WHERE E.EID = R.EID
AND R.CID = C.CID
AND C.NAME = 'OBAMA';

NOW YOU CAN CONVERT THIS QUERY TO RELATIONAL ALGEBRA.

回答2:

That is correct, though without putting the table aliases (R, C and E) against their respective tables it is unlikely to run.

There is a new form for expressing joins which is more common these days:

SELECT E.eid
FROM Customer C
INNER JOIN Resp_for R
    ON R.cid = C.cid
INNER JOIN Employee E
    ON E.eid = R.eid
WHERE C.name = 'Obama'

Same meaning, different format and IMHO easier to assimilate.

来源：https://stackoverflow.com/questions/23781004/selecting-from-three-relation-with-relational-algebra