Different database queries
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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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computational-mathematics
edited Nov 21 at 10:57
Somos
12.7k11034
asked Nov 21 at 10:44
Krister
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up vote
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down vote
favorite
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 at 10:57
Somos
12.7k11034
asked Nov 21 at 10:44
Krister
1
add a comment |
up vote
0
down vote
favorite
up vote
0
down vote
favorite
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 at 10:57
Somos
12.7k11034
asked Nov 21 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 at 10:57
Somos
12.7k11034
asked Nov 21 at 10:44
Krister
1
edited Nov 21 at 10:57
Somos
12.7k11034
asked Nov 21 at 10:44
Krister
1
edited Nov 21 at 10:57
Somos
12.7k11034
edited Nov 21 at 10:57
Somos
12.7k11034
edited Nov 21 at 10:57
Somos
12.7k11034
12.7k11034
asked Nov 21 at 10:44
Krister
1
asked Nov 21 at 10:44
Krister
1
asked Nov 21 at 10:44
Krister
1
1
add a comment |
add a comment |
2 Answers
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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 $X$.
answered Nov 21 at 11:27
Berci
59k23671
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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.
answered Nov 21 at 11:30
Ingix
2,999145
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
0
down vote
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 at 11:27
Berci
59k23671
add a comment |
up vote
0
down vote
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 at 11:27
Berci
59k23671
add a comment |
up vote
0
down vote
up vote
0
down vote
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 at 11:27
Berci
59k23671
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 at 11:27
Berci
59k23671
answered Nov 21 at 11:27
Berci
59k23671
answered Nov 21 at 11:27
Berci
59k23671
answered Nov 21 at 11:27
Berci
59k23671
59k23671
add a comment |
add a comment |
up vote
0
down vote
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 at 11:30
Ingix
2,999145
add a comment |
up vote
0
down vote
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 at 11:30
Ingix
2,999145
add a comment |
up vote
0
down vote
up vote
0
down vote
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 at 11:30
Ingix
2,999145
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 at 11:30
Ingix
2,999145
answered Nov 21 at 11:30
Ingix
2,999145
answered Nov 21 at 11:30
Ingix
2,999145
answered Nov 21 at 11:30
Ingix
2,999145
2,999145
add a comment |
add a comment |
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