Mixing
Different database queries
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
edited Nov 21 '18 at 10:57
Somos
13k11034
asked Nov 21 '18 at 10:44
Krister
1
add a comment |
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
edited Nov 21 '18 at 10:57
Somos
13k11034
asked Nov 21 '18 at 10:44
Krister
1
add a comment |
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
edited Nov 21 '18 at 10:57
Somos
13k11034
asked Nov 21 '18 at 10:44
Krister
1
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
edited Nov 21 '18 at 10:57
Somos
13k11034
asked Nov 21 '18 at 10:44
Krister
1
edited Nov 21 '18 at 10:57
Somos
13k11034
asked Nov 21 '18 at 10:44
Krister
1
edited Nov 21 '18 at 10:57
Somos
13k11034
edited Nov 21 '18 at 10:57
Somos
13k11034
edited Nov 21 '18 at 10:57
Somos
13k11034
13k11034
asked Nov 21 '18 at 10:44
Krister
1
asked Nov 21 '18 at 10:44
Krister
1
asked Nov 21 '18 at 10:44
Krister
1
1
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.
answered Nov 21 '18 at 11:27
Berci
59.7k23672
add a comment |
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.
answered Nov 21 '18 at 11:30
Ingix
3,334145
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.
answered Nov 21 '18 at 11:27
Berci
59.7k23672
add a comment |
a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.
answered Nov 21 '18 at 11:27
Berci
59.7k23672
add a comment |
a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.
answered Nov 21 '18 at 11:27
Berci
59.7k23672
a) says that for every id $Y$, the name matches $X$, i.e. this set contain $X$ only if all names were equal to $X$.
answered Nov 21 '18 at 11:27
Berci
59.7k23672
answered Nov 21 '18 at 11:27
Berci
59.7k23672
answered Nov 21 '18 at 11:27
Berci
59.7k23672
answered Nov 21 '18 at 11:27
Berci
59.7k23672
59.7k23672
add a comment |
add a comment |
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.
answered Nov 21 '18 at 11:30
Ingix
3,334145
add a comment |
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.
answered Nov 21 '18 at 11:30
Ingix
3,334145
add a comment |
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.
answered Nov 21 '18 at 11:30
Ingix
3,334145
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.
answered Nov 21 '18 at 11:30
Ingix
3,334145
answered Nov 21 '18 at 11:30
Ingix
3,334145
answered Nov 21 '18 at 11:30
Ingix
3,334145
answered Nov 21 '18 at 11:30
Ingix
3,334145
3,334145
add a comment |
add a comment |
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