考虑以下关系模式。
Students(rollno: integer, sname: string)
Courses(courseno: integer, cname: string)
Registration(rollno: integer, courseno: integer, percent: real)以下哪个查询与该英语查询等效?
"Find the distinct names of all students who score
more than 90% in the course numbered 107"
(A)一，二，三和四
(B)仅I，II和III
(C)仅I，II和IV
(D)仅II，III和IV答案： (A)
解释：
Option A:
This is a SQL query expression. It first perform a cross product of Students
and Registration, then WHERE clause only keeps those rows in the cross product
set where the student is registered for course no 107, and percentage is > 90.
Then select distinct statement gives the distinct names of those students as the
result set.
Option B:
This is a relational algebra expression. It first perform a NATURAL JOIN
of Students and Registration (NATURAL JOIN implicitly joins on the basis
of common attribute, which here is rollno ), then the select operation( sigma)
keeps only those rows where the student is registered for courseno 107,
and percentage is > 90. And then the projection operation (pi) projects only
distinct student names from the set.
Note: Projection operation (pi) always gives the distinct result.
Option C:
This is a Tuple Relational Calculus (TRC) language expression,
It is not a procedural language (i.e. it only tells “what to do”,
not “how to do”). It just represents a declarative mathematical
expression.
Here T is a Tuple variable.
From left to right, it can be read like this, “It is a set of
tuples T, where, there exists a tuple S in Relation Students, and
there exist a tuple R in relation Registration, such that
S.rollno = R.rollno AND R.couseno = 107 AND R.percent > 90 AND
T.sname = S.sname”. And the schema of this result is (sname), i.e. each
tuple T will contain only student name, because only T.sname has been defined
in the expression.
As TRC is a mathematical expression, hence it is expected to give only distinct result set.
Option D:
This is a Domain Relational Calculus (DRC) language expression.
This is also not procedural. Here SN is a Domain Variable. It can be read
from left to right like this “The set of domain variable SN, where,
there exist a domain variable SR , and a domain variable Rp, such that,
SN and SR domain variables is in relation Students and SR,107,RP is a domain
variables set in relation Registration, AND RP > 90 “
Above, SN represents sname domain attribute in Students relation, SR
represents rollno domain attribute in Students relation, and RP represents
percentage domain attribute in Registration relation.
The schema for the result set is (SN), i.e. only student name.
As DRC is a mathematical expression, hence it is expected to
give only distinct result set.
这个问题的测验