How to find the maximum in relational algebra?












0












$begingroup$


This is my database course's homework.
We have this relation.
Trained(A, B) A trained B



How can I figure out who has trained the most people by using relational algebra or it cannot be expressed in relational algebra?
for example,
(John, Kenny)
(John, Kei)
(John, Cohen)
(Willson, John)
(Kenny, Peter)
(Ho, Tina)



In these tables, the answer obviously is John.But is it possible to express in relational algebra?










share|cite|improve this question









$endgroup$

















    0












    $begingroup$


    This is my database course's homework.
    We have this relation.
    Trained(A, B) A trained B



    How can I figure out who has trained the most people by using relational algebra or it cannot be expressed in relational algebra?
    for example,
    (John, Kenny)
    (John, Kei)
    (John, Cohen)
    (Willson, John)
    (Kenny, Peter)
    (Ho, Tina)



    In these tables, the answer obviously is John.But is it possible to express in relational algebra?










    share|cite|improve this question









    $endgroup$















      0












      0








      0





      $begingroup$


      This is my database course's homework.
      We have this relation.
      Trained(A, B) A trained B



      How can I figure out who has trained the most people by using relational algebra or it cannot be expressed in relational algebra?
      for example,
      (John, Kenny)
      (John, Kei)
      (John, Cohen)
      (Willson, John)
      (Kenny, Peter)
      (Ho, Tina)



      In these tables, the answer obviously is John.But is it possible to express in relational algebra?










      share|cite|improve this question









      $endgroup$




      This is my database course's homework.
      We have this relation.
      Trained(A, B) A trained B



      How can I figure out who has trained the most people by using relational algebra or it cannot be expressed in relational algebra?
      for example,
      (John, Kenny)
      (John, Kei)
      (John, Cohen)
      (Willson, John)
      (Kenny, Peter)
      (Ho, Tina)



      In these tables, the answer obviously is John.But is it possible to express in relational algebra?







      relation-algebra






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Oct 10 '16 at 0:40









      JohnJohn

      24




      24






















          2 Answers
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          0












          $begingroup$

          You use GROUP and HAVING. I don't know how your class does notation, but in mine we do something like this:




          SELECT A FROM Trained GROUP BY A HAVING COUNT(A)




          I'm not sure if this is correct, but it should be similar.






          share|cite|improve this answer











          $endgroup$





















            0












            $begingroup$

            Yes, it is possible to express it in relational algebra (RA).



            The following line groups column A by COUNT(A). And plots COUNT(A) as 'frequencyOf_A'.



            γ A; COUNT(A)→frequencyOf_A (Trained)



            Resulting in this:



            Trained.A frequencyOf_A
            John 3
            Willson 1
            Kenny 1
            Ho 1


            I used this RA engine. And uploaded your relation via github's gist. Thanks. (On the side "relation algebra" is mathematics, while "relational algebra" is considered computer science I think.)






            share|cite|improve this answer











            $endgroup$













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              2 Answers
              2






              active

              oldest

              votes








              2 Answers
              2






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes









              0












              $begingroup$

              You use GROUP and HAVING. I don't know how your class does notation, but in mine we do something like this:




              SELECT A FROM Trained GROUP BY A HAVING COUNT(A)




              I'm not sure if this is correct, but it should be similar.






              share|cite|improve this answer











              $endgroup$


















                0












                $begingroup$

                You use GROUP and HAVING. I don't know how your class does notation, but in mine we do something like this:




                SELECT A FROM Trained GROUP BY A HAVING COUNT(A)




                I'm not sure if this is correct, but it should be similar.






                share|cite|improve this answer











                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  You use GROUP and HAVING. I don't know how your class does notation, but in mine we do something like this:




                  SELECT A FROM Trained GROUP BY A HAVING COUNT(A)




                  I'm not sure if this is correct, but it should be similar.






                  share|cite|improve this answer











                  $endgroup$



                  You use GROUP and HAVING. I don't know how your class does notation, but in mine we do something like this:




                  SELECT A FROM Trained GROUP BY A HAVING COUNT(A)




                  I'm not sure if this is correct, but it should be similar.







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Oct 23 '16 at 17:01

























                  answered Oct 23 '16 at 16:56









                  user1766555user1766555

                  3092618




                  3092618























                      0












                      $begingroup$

                      Yes, it is possible to express it in relational algebra (RA).



                      The following line groups column A by COUNT(A). And plots COUNT(A) as 'frequencyOf_A'.



                      γ A; COUNT(A)→frequencyOf_A (Trained)



                      Resulting in this:



                      Trained.A frequencyOf_A
                      John 3
                      Willson 1
                      Kenny 1
                      Ho 1


                      I used this RA engine. And uploaded your relation via github's gist. Thanks. (On the side "relation algebra" is mathematics, while "relational algebra" is considered computer science I think.)






                      share|cite|improve this answer











                      $endgroup$


















                        0












                        $begingroup$

                        Yes, it is possible to express it in relational algebra (RA).



                        The following line groups column A by COUNT(A). And plots COUNT(A) as 'frequencyOf_A'.



                        γ A; COUNT(A)→frequencyOf_A (Trained)



                        Resulting in this:



                        Trained.A frequencyOf_A
                        John 3
                        Willson 1
                        Kenny 1
                        Ho 1


                        I used this RA engine. And uploaded your relation via github's gist. Thanks. (On the side "relation algebra" is mathematics, while "relational algebra" is considered computer science I think.)






                        share|cite|improve this answer











                        $endgroup$
















                          0












                          0








                          0





                          $begingroup$

                          Yes, it is possible to express it in relational algebra (RA).



                          The following line groups column A by COUNT(A). And plots COUNT(A) as 'frequencyOf_A'.



                          γ A; COUNT(A)→frequencyOf_A (Trained)



                          Resulting in this:



                          Trained.A frequencyOf_A
                          John 3
                          Willson 1
                          Kenny 1
                          Ho 1


                          I used this RA engine. And uploaded your relation via github's gist. Thanks. (On the side "relation algebra" is mathematics, while "relational algebra" is considered computer science I think.)






                          share|cite|improve this answer











                          $endgroup$



                          Yes, it is possible to express it in relational algebra (RA).



                          The following line groups column A by COUNT(A). And plots COUNT(A) as 'frequencyOf_A'.



                          γ A; COUNT(A)→frequencyOf_A (Trained)



                          Resulting in this:



                          Trained.A frequencyOf_A
                          John 3
                          Willson 1
                          Kenny 1
                          Ho 1


                          I used this RA engine. And uploaded your relation via github's gist. Thanks. (On the side "relation algebra" is mathematics, while "relational algebra" is considered computer science I think.)







                          share|cite|improve this answer














                          share|cite|improve this answer



                          share|cite|improve this answer








                          edited Oct 18 '17 at 0:14

























                          answered Oct 17 '17 at 23:44









                          Petr KosvanecPetr Kosvanec

                          11




                          11






























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