Different database queries












0














We have the table person (pid,pname)
We want to find the names of the persons. What is difference between a and d? I know that d is the correct answer but I don't understand why.



$$a), {langle Xrangle mid forall Y, (textrm{person}(Y,X))}.$$
$$d), {langle Xranglemid exists Y, (textrm{person}(Y,X))}.$$



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    0














    We have the table person (pid,pname)
    We want to find the names of the persons. What is difference between a and d? I know that d is the correct answer but I don't understand why.



    $$a), {langle Xrangle mid forall Y, (textrm{person}(Y,X))}.$$
    $$d), {langle Xranglemid exists Y, (textrm{person}(Y,X))}.$$



    original image






    computational-mathematics







    share|cite|improve this question



























      0












      0








      0







      We have the table person (pid,pname)
      We want to find the names of the persons. What is difference between a and d? I know that d is the correct answer but I don't understand why.



      $$a), {langle Xrangle mid forall Y, (textrm{person}(Y,X))}.$$
      $$d), {langle Xranglemid exists Y, (textrm{person}(Y,X))}.$$



      original image






      computational-mathematics







      share|cite|improve this question















      We have the table person (pid,pname)
      We want to find the names of the persons. What is difference between a and d? I know that d is the correct answer but I don't understand why.



      $$a), {langle Xrangle mid forall Y, (textrm{person}(Y,X))}.$$
      $$d), {langle Xranglemid exists Y, (textrm{person}(Y,X))}.$$



      original image






      computational-mathematics




      computational-mathematics






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      edited Nov 21 '18 at 10:57









      Somos

      13k11034




      13k11034










      asked Nov 21 '18 at 10:44









      Krister

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          a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.






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            a) would give you the list of all pnames (X) that are are associated will all pids (notice the "for all" ($forall$) quantifier before the Y). Assuming that pids are unique (they usually are keys), this would give you an empty list if you had 2 different persons in the list.



            d) gives you the list of all existing pnames (X), because it is only required that at least one pid exists ($exists$ before the Y) such that (Y,X) is in the database. In your database, each entry must be of the form (pid, pname), so each existing pname must have (at least one) pid that occurs with it, so query d) finds it.






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              2 Answers
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              2 Answers
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              active

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              0














              a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.






              share|cite|improve this answer


























                0














                a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.






                share|cite|improve this answer
























                  0












                  0








                  0






                  a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.






                  share|cite|improve this answer












                  a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Nov 21 '18 at 11:27









                  Berci

                  59.7k23672




                  59.7k23672























                      0














                      a) would give you the list of all pnames (X) that are are associated will all pids (notice the "for all" ($forall$) quantifier before the Y). Assuming that pids are unique (they usually are keys), this would give you an empty list if you had 2 different persons in the list.



                      d) gives you the list of all existing pnames (X), because it is only required that at least one pid exists ($exists$ before the Y) such that (Y,X) is in the database. In your database, each entry must be of the form (pid, pname), so each existing pname must have (at least one) pid that occurs with it, so query d) finds it.






                      share|cite|improve this answer


























                        0














                        a) would give you the list of all pnames (X) that are are associated will all pids (notice the "for all" ($forall$) quantifier before the Y). Assuming that pids are unique (they usually are keys), this would give you an empty list if you had 2 different persons in the list.



                        d) gives you the list of all existing pnames (X), because it is only required that at least one pid exists ($exists$ before the Y) such that (Y,X) is in the database. In your database, each entry must be of the form (pid, pname), so each existing pname must have (at least one) pid that occurs with it, so query d) finds it.






                        share|cite|improve this answer
























                          0












                          0








                          0






                          a) would give you the list of all pnames (X) that are are associated will all pids (notice the "for all" ($forall$) quantifier before the Y). Assuming that pids are unique (they usually are keys), this would give you an empty list if you had 2 different persons in the list.



                          d) gives you the list of all existing pnames (X), because it is only required that at least one pid exists ($exists$ before the Y) such that (Y,X) is in the database. In your database, each entry must be of the form (pid, pname), so each existing pname must have (at least one) pid that occurs with it, so query d) finds it.






                          share|cite|improve this answer












                          a) would give you the list of all pnames (X) that are are associated will all pids (notice the "for all" ($forall$) quantifier before the Y). Assuming that pids are unique (they usually are keys), this would give you an empty list if you had 2 different persons in the list.



                          d) gives you the list of all existing pnames (X), because it is only required that at least one pid exists ($exists$ before the Y) such that (Y,X) is in the database. In your database, each entry must be of the form (pid, pname), so each existing pname must have (at least one) pid that occurs with it, so query d) finds it.







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Nov 21 '18 at 11:30









                          Ingix

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