The Solutions to the Midterm of CMPUT 391 Fall term, 2005 1. Give ...

The Solutions to the Midterm of CMPUT 391
Fall term, 2005
1. Give a schedule, as a sequence of database access operations, with at least two transactions
that observes the repeatable read isolation level but not the (anomaly)serializable isolation level.
Each operation, with a transaction id attached, may be of the following forms:
• READ(T i , A),
• WRITE(T i , A),
• READ LOCK(T i , A),
• WRITE LOCK(T i , A),
• an SQL statement by T i .
Specify any reasonable assumptions if needed. [10]
Solution Assume the database contains a table sample with the folowing rows:
number
10
20
30
The following schedule observes REPEATABLE READ but not SERIALIABLE.
T 1 T 2
READ LOCK(number(10)
READ LOCK(number(20)
SELECT * FROM sample WHERE number

employee( e_id, e_name, position, salary )
works( e_id, d_id )
dept( d_id, budget, manager_id )
The first table indicates the job title and salary for each employee, the second shows the department
an employee works for, and the last one indicates the budget and boss of each department.
Write one or more triggers to ensure that no employee’s salary is higher than that of the manager
of the department he/she is working for. [20]
Solution This constraint can only be imposed by three triggers, one for each table, whenever
an update on or an insertion to the table occurs. For simplicity, one trigger on employee is
presented below, and the other two are similart.
CREATE TRIGGER salary_cap
BEFORE INSERT OR UPDATE ON employee
FOR EACH ROW
BEGIN
SELECT e.salary INTO salary
FROM employee e, works w, dept d
WHERE w.e_id = new.e_id AND
w.d_id = d.d_id AND
manager_id = e.e_id;
END
IF (salary > NEW.salary )
THEN raise_application_error(-2000, ’Salary is too high’);
ELSE IF
4. Consider the following functional dependencies over the attribute set R = ABCDEF GH:
A → E
AD → BE
AC → E
E → B
BG → F
BE → D
BDH → E
F → A
D → H
CD → A
(a) Find a minimal cover of the given set of FDs.
(b) Find a join loss-less, dependency preserving and 3NF decomposition of R.
(c) Check if all the resulting relations in the previous step are in BCNF. If you find a schema
that is not, decompose it into a lossless BCNF.
Explain all steps. [20]
Solution
(a) The following three steps lead to a minimal cover.
Right-reducing It is done by split AD → BE into AD → B and AD → E.
Left-reducing leads to the following set:
2

A → E BD → E
A → B F → A
E → B D → H
BG → F CD → A
E → D
By eliminating redundant ones, we obtain the following minimal cover of the given set of
FDs:
A → E BD → E
E → B F → A
BG → F D → H
E → D CD → A
(b) The following is a join lossless, dependency preserving and 3NF decomposition of the original
database schema.
AE, EBD, BGF, FA, DH, CDA, BGC.
Note that BFC is a candicate key that was added later.
(c) All tables, except CDA, in the above database schema are in BCNF. CDA is not in BCNF
because A → D but not A → C. CDA can be decomposited into {AD, AC}.
5. Consider a university registration database with the following tables:
student(sid, sname, address,city,status)
professor(pid, pname, deptid )
course( crscode, title, description )
transcript( tr sid, tr crscode, grade)
teaching( t pid, t crscode )
with the obvious meanings.
Assume that the Web server delivers the contents of these relations using the following XML
format:
the name of the relation is the top-level element under root, each tuple is represented as a
table-nameed tuple element, and each relation attribute is represented as an empty element that
has a value attribute.
For instance, the student table would be represented as followings:
.
.
.
3

.
.
.
.
Formula the following queries using XQuery:
• Produce the list of all students who live at Edmonton.
FOR $s IN document(‘‘registrantion.cml’’)/root/student/
WHERE $s/city@value = ‘‘Edmonton’’]
RETURN
@s
• List the titles of all the CS courses. A CS course is a course taught by a professor working
for the department with deptid = ’CS’. [20]
FOR $t IN document(‘‘registrantion.cml’’)/teaching/teaching_tuple,
$c IN document(‘‘registrantion.cml’’)/course/course_tuple,
$p IN document(‘‘registrantion.cml’’)/professor/professor_tuple
WHERE $p/deptid/@value = ‘‘CS’’ AND
$t/t_pid/@value = $p/pid/@value AND
$t/t_crscode/@value = $c/coscode/@value
RETURN
@c/title
4