Suppose we have a relational database with five tables. table key Attributes S(sid, A) Sid T(tid, B) Tid U(uid, C) Uid R(sid, tid, D) sid, tid Q(tid, uid, E)
Suppose we have a relational
table key Attributes
S(sid, A) Sid
T(tid, B) Tid
U(uid, C) Uid
R(sid, tid, D) sid, tid
Q(tid, uid, E) tid, uid
Here R implements a many-to-many relationship between the entities implemented with tables S
and T, and Q implements a many-to-many relationship between the entities implemented with
tables T and U.
D. Suppose you replaced SELECT with SELECT DISTINCT in your queries from parts (a) and
Could one of these modified queries return more records than the other? If so, which one?
Justify your answer.
E. Consider again your query from part (a). If pair sid, uid is returned by this query then there
must exist at least one “path” that goes from from table S to table T (via relation R) and then
from table T to table U (via relation Q).
Note that there can be many such paths for a given pair sid, uid. Write an SQL query that
returns records of the form tid, total where tid is a key of a record from table T and total
indicates the total number of such paths that “go through” that record.
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