DatabaseManagementSystem
Question 1 
What does the above expression generate?
A  Employee numbers of all employees whose age is not the maximum. 
B  Employee numbers of only those employees whose age is the maximum. 
C  Employee numbers of all employees whose age is not the minimum. 
D  Employee numbers of only those employees whose age is more than the age of exactly one other employee. 
Question 2 
S_{1}: r_{1}(x) r_{1}(y) r_{2}(x) r_{2}(y) w_{2}(y) w_{1}(x)
S_{2}: r_{1}(x) r_{2}(x) r_{2}(y) w_{2}(y) r_{1}(y) w_{1}(x)
Which one of the following options is correct?
A  S_{1}is not conflict serializable, and S_{2} is conflict serializable. 
B  Neither S_{1}nor S_{2}is conflict serializable.

C  Both S_{1}and S_{2}are conflict serializable. 
D  S_{1}is conflict serializable, and S_{2}is not conflict serializable. 
Question 3 
Consider the decomposition of the relation R into the consistent relations according to the following two decomposition schemes.
D_{1}: R=[(P,Q,S,T); (P,T,X); (Q,Y); (Y,Z,W)]
D_{2}: R=[(P,Q,S);(T,X);(Q,Y);(Y,Z,W)]
Which one of the following options is correct?
A  D^{1}is a lossy decomposition, but D^{2}is a lossless decomposition.

B  Both D^{1}and D^{2}are lossy decompositions. 
C  Both D^{1}and D^{2}are lossless decompositions. 
D  D^{1}is a lossless decomposition, but D^{2}is a lossy decomposition. 
Given functional dependencies set:
PQ>X
P>YX
Q>Y
Y>ZW
 While merging the tables there should be some common attribute(s) and it should be a candidate key of one of the tables.
 R1 should be merged with R2 because PT is a key of R2.
 R3 should be merged with PQSTX because Q is a key of R3.
 R4 should be merged with PQSTXY because Y is a key of R4.
 R1 should be merged with R3 because Q is a key of R3.
 R4 should be merged with PQSY because Y is a key of R4.
 Now, there is no common attribute in between R2(TX) and PQSYZW.
 Hence, D2 is lossy decomposition.
Question 4 
A  The same undo and redo list will be used while recovering again. 
B  The database will become inconsistent. 
C  The system cannot recover any further. 
D  All the transactions that are already undone and redone will not be recovered again. 
Question 5 
A  820 
Explanation :
Probability of 1st condition being satisfied(say P(A)) = 10/15 = 2/3
Probability of 2nd condition being satisfied(say P(B)) = 1/20
Probability of both conditions being satisfied(say P(A intersection B)) = 2/3*1/20 = 1/30
Probability of any one condition being satisfied = P(A union B) = P(A)+P(B)P(A intersection B) = 2/3 + 1/20  1/30 = 41/60
therefore, expected number of tuples = (41/60)*1200 = 820
Question 6 
(a) Consider the relation scheme R(A, B, C) with the following functional dependencies:
A, B → C, C → A
Show that the scheme R is the Third Normal Form (3NF) but not in BoyceCode Normal Form (BCNF).
(b) Determine the minimal keys of relation R.
A  Theory Explanation. 
Question 7 
Consider the relation scheme.
AUTHOR (ANAME, INSTITUTION, ACITY, AGE) PUBLISHER (PNAME, PCITY) BOOK (TITLE, ANAME, PNAME)
Express the following queries using (one or more of )SELECT, PROJECT, JOIN and DIVIDE operations.
(a) Get the names of all publishers.
(b) Get values of all attributes of all authors who have published a book for the
publisher with PNAME = ‘TECHNICAL PUBLISHERS’.
(c) Get the names of all authors who have published a book for any publisher located in Madras.
A  Theory Explanation. 
Question 8 
Consider a relational database containing the following schemas.
The primary key of each table is indicated by underlying the constituent fields.
SELECT s.sno, s.sname FROM Suppliers s, Catalogue c WHERE s.sno = c.sno AND Cost > (SELECT AVG (cost) FROM Catalogue WHERE pno = ‘P4’ GROUP BY pno);
The number of rows returned by the above SQL query is
A  0 
B  5 
C  4 
D  2 
AVG(COST)

225
The outer query “select s.sno, s.sname from suppliers s, catalogue c where s.sno=c.sno” returns:
SNO SNAME

S1 M/s Royal furniture
S1 M/s Royal furniture
S1 M/s Royal furniture
S2 M/s Balaji furniture
S2 M/s Balaji furniture
S3 M/s Premium furniture
S3 M/s Premium furniture
S3 M/s Premium furniture
S3 M/s Premium furniture
So, the final result of the query is:
SN SNAME

S2 M/s Balaji furniture
S3 M/s Premium furniture
S3 M/s Premium furniture
S3 M/s Premium furniture
Therefore, 4 rows will be returned by the query.
Question 9 
Which one of the following is used to represent the supporting manyone relationships of a weak entity set in an entityrelationship diagram?
A  Ovals that contain underlined identifiers

B  Rectangles with double/bold border 
C  Diamonds with double/bold border

D  Ovals with double/bold border

Question 10 
Consider a schedule of transactions T_{1} and T_{2}:
Here, RX stands for “Read(X)” and WX stands for “Write(X)”. Which one of the following schedules is conflict equivalent to the above schedule?
A  
B  
C  
D 
• First, let’s list the conflict operations of each of the schedule given in the options and compare with the conflict operations of schedule which is given in the question.
Given schedule:
Conflict operations:
R2(B) → W1(B)
W2(B) → W1(B)
R1(C) → W2(C)
R2(D) → W1(D)
Option(1):
Conflict operations:
R1(C) → W2(C)
W1(D) → R2(D)
W1(B) → R2(B)
W1(B) → W2(B)
Option(2):
Conflict operations:
R2(B) → W1(B)
W2(B) → W1(B)
R2(D) → W1(D)
R1(C) → W2(C)
Option(3):
Conflict operations:
R2(B) → W1(B)
W2(B) → W1(B)
R2(D) → W1(D)
W2(C) → R1(C)
Option(4):
Conflict operations:
R1(C) → W2(C)
W1(D) → R2(D)
R2(B) → W1(B)
W2(B) → W1(B)
The conflict operations in the option (2) and given schedule are appearing in the same sequence order, so option (2) is the answer.
Question 11 
Consider a relational table R that is in 3NF, but not in BCNF. Which one of the following statements is TRUE?
A  A cell in R holds a set instead of an atomic value. 
B  R has a nontrivial functional dependency X→A, where X is not a superkey and A is a nonprime attribute and X is not a proper subset of any key.

C  R has a nontrivial functional dependency X→A, where X is not a superkey and A is a nonprime attribute and X is a proper subset of some key. 
D  R has a nontrivial functional dependency X→A, where X is not a superkey and A is a prime attribute. 
FDs:
AB → C
BC → A
(BD)^{+} = BD ✖
(ABD)^{+} = ABDC ✔
(CBD)^{+} = CBDA ✔
Candidate keys = {ABD, CBD}
• The relation R is in 3NF, as there are no transitive dependencies.
• The relation R is not in BCNF, because the left side of both the FD’s are not Super keys.
• In R, BC → A is a nontrivial FD and in which BC is not a Super key and A is a prime attribute.
Question 12 
Consider a database implemented using B+ tree for file indexing and installed on a disk drive with block size of 4 KB. The size of search key is 12 bytes and the size of tree/disk pointer is 8 bytes. Assume that the database has one million records. Also assume that no node of the B+ tree and no records are present initially in main memory. Consider that each record fits into one disk block. The minimum number of disk accesses required to retrieve any record in the database is ______.
A  4 
(1) Database BF = 1
No. of block = 10^{6} } ➝ 1 block access from database
(2) ⎡10^{6}/204⎤ = 491
(3) ⎡491/204⎤ = 3
(4) ⎡3/204⎤ = 1
So, 1+3 = 4 disk accesses are required to retrieve any record in the database.
Question 13 
A library relational database system uses the following schema
USERS (User#, UserName, HomeTown) BOOKS (Book#, BookTitle, AuthorName) ISSUED (Book#, User#, Date)
Explain in one English sentence, what each of the following relational algebra queries is designed to determine
(a) σ User #=6 (11 User #, Book Title ((USERS ISSUED) BOOKS)) (b) σ Author Name (BOOKS (σ Home Town) = Delhi (USERS ISSUED)))
A  Theory Explanation. 
Question 14 
For a database relation R(a,b,c,d), where the domains a, b, c, d include only atomic values, only the following functional dependencies and those that can be inferred from them hold:
a → c b → d
This relation is
A  in first normal form but not in second normal form 
B  in second normal form but not in third normal form 
C  in third normal form 
D  None of the above 
Since all a, b, c, d are atomic. So the relation is in 1NF.
Checking the FD's
a → c
b → d
We can see that there is partial dependencies. So it is not 2NF.
So answer is option (A).
Question 15 
Let R(a,b,c) and S(d,e,f) be two relations in which d is the foreign key of S that refers to the primary key of R. Consider the following four operations R and S
(a) Insert into R (b) Insert into S (c) Delete from R (d) Delete from S
Which of the following can cause violation of the referential integrity constraint above?
A  None of (a), (b), (c) or (d) can cause its violation 
B  All of (a), (b), (c) and (d) can cause its violation 
C  Both (a) and (d) can cause its violation 
D  Both (b) and (c) can cause its violation 
Here 'd' is the foreign key of S and let 'a' is the primary key of R.
(A) Insertion into R: will cause no violation.
(B) Insertion into S: may cause violation because there may not be entry of the tuple in relation R. Example entry of 〈S_{4}, __, __〉 is not allowed.
(C) Delete from R: may cause violation. For example, deletion of tuple 〈S_{2}, __, __〉 will cause violation as there is entry of S_{2} in the foreign key table.
(D) Delete from S: will cause no violation as it does not result inconsistency.
Question 16 
Match the pairs in the following questions:
(a) Secondary index (p) Function dependency (b) Nonprocedural query language (q) Btree (c) Closure of a set of attributes (r) Domain calculus (d) Natural join (s) Relational algebraic operations
A  (a)  (q), (b)  (r), (c)  (p), (d)  (s) 
Nonprocedural query language → Domain calculus
Closure of set of attributes → Function dependency
Natural join → Relational algebraic operations
Question 17 
Choose the correct alternatives (More than one may be correct).
Indicate which of the following statements are true: A relational database which is in 3NF may still have undesirable data redundancy because there may exist:
A  Transitive functional dependencies. 
B  Nontrivial functional dependencies involving prime attributes on the rightside.

C  Nontrivial functional dependencies involving prime attributes only on the leftside.

D  Nontrivial functional dependencies involving only prime attributes. 
E  Both (B) and (D). 
B) 3NF because right side is prime attribute.
C) Not in 3NF, because lets suppose ABC is a candidate key. Now consider
AB → Nonprime attribute
which show it is not in 3NF
D) Involves only prime attribute, so right side should definitely contain only prime attribute. So in 3NF.
Question 18 
Given two union compatible relations R_{1}(A,B) and R_{2}(C,D), what is the result of the operation R_{1}A = CAB = DR_{2}?
A  R_{1} ∪ R_{2} 
B  R_{1} × R_{2} 
C  R_{1}  R_{2} 
D  R_{1} ∩ R_{2} 
Question 19 
Which normal form is considered adequate for normal relational database design?
A  2 NF 
B  5 NF 
C  4 NF 
D  3 NF 
Question 20 
There are 5 records in a database.
There is an index file associated with this and it contain the values 1, 3, 2, 5 and 4. Which one of the fields is the index built form?
A  Age 
B  Name 
C  Occupation 
D  Category 
Question 21 
Which of the following query transformations (i.e. replacing the l.h.s. expression by the r.h.s. expression) is incorrect? R_{1} and R_{2} are relations, C_{1}, C_{2} are selection conditions and A_{1}, A_{2} are attributes of R_{1}?
A  σ_{C1}(σ_{C1}(R_{1})) → σ_{C2}(σ_{C2}(R_{1})) 
B  σ_{C1}(σ_{A1}(R_{1})) → σ_{A1}(σ_{C1}(R_{1})) 
C  σ_{C1}(R_{1} ∪ R_{2}) → σ_{C1}(R_{1}) ∪ σ_{C1} 
D  π_{A1}(σ_{C1}(R_{1})) → σ_{C1}(σ_{A1}(R_{1})) 
Question 22 
(a) Suppose we have a database consisting of the following three relations.
FREQUENTS(student, parlor) giving the parlors each student visits.
SERVES(parlor, icecream) indicating what kind of icecreams each parlor serves.
LIKES(student, icecream) indicating what icecreams each parlor serves.
(Assuming that each student likes at least one icecream and frequents at least one parlor)
Express the following in SQL:
Print the students that frequent at least one parlor that serves some icecream that they like.
(b) In a computer system where the 'bestfit' algorithm is used for allocating 'jobs' to 'memory partitions', the following situation was encountered:
When will the 20K job complete? Note  This question was subjective type.
A  Theory Explanation. 
Question 23 
(a) Four jobs are waiting to be run. Their expected run times are 6, 3, 5 and x. In what order should they be run to minimize the average response time?
(b) Write a concurrent program using par begin  par end to represent the precedence graph shown below.
A  Theory Explanation. 
Question 24 
Consider the following database relations containing the attributes
Book_id Subject_Category_of_book Name_of_Author Nationality_of_Author with Book_id as the Primary Key.
(a) What is the highest normal form satisfied by this relation?
(b) Suppose the attributes Book_title and Author_address are added to the relation, and the primary key is changed to (Name_of_Author, Book_Title), what will be the highest normal form satisfied by the relation?
A  Theory Explanation. 
Question 25 
Consider the following relational database schemes:
COURSES(Cno, name) PREREQ(Cno, pre_Cno) COMPLETED(student_no, Cno)
COURSES give the number and the name of all the available courses.
PREREQ gives the information about which course are prerequisites for a given course.
COMPLETED indicates what courses have been completed by students.
Express the following using relational algebra:
List all the courses for which a student with student_no 2310 has completed all the prerequisites.
A  Theory Explanation. 
Question 26 
Consider the join of a relation R with a relation S. If R has m tuples and S has n tuples then the maximum and minimum sizes of the join respectively are
A  m + n and 0 
B  mn and 0 
C  m + n and m – n 
D  mn and m + n 
Suppose there is no common attribute in R and S due to which natural join will act as cross product. So then in cross product total no. of tuples will be mn.
For minimum:
Suppose there is common attribute in R and S, but none of the row of R matches with rows of S then minimum no. of tuples will be 0.
Question 27 
The relational algebra expression equivalent to the following tuple calculus expression:
{t t ∈ r ∧(t[A] = 10 ∧ t[B] = 20)} isA  σ_{(A=10∨B=20)} (r) 
B  σ_{(A=10)} (r) ∪ σ_{(B=20)} (r) 
C  σ_{(A=10)} (r) ∩ σ_{(B=20)} (r) 
D  σ_{(A=10)} (r)  σ_{(B=20)} (r) 
σ_{(A=10)} (r) ∩ σ_{(B=20)} (r)
Question 28 
Let R = (A, B, C, D, E, F) be a relation scheme with the following dependencies: C→F, E→A, EC→D, A→B. Which of the following is a key of R?
A  CD 
B  EC 
C  AE 
D  AC 
A) (CD)^{+} = cdf
Not a key.
B) (EC)^{+} = ecdabf
Yes, it is a key.
C) (AE)^{+} = aeb
Not a key. D) (AC)^{+} = abcf
Not a key.
Question 29 
Which of the following is correct?
A  Btrees are for storing data on disk and B^{+} trees are for main memory. 
B  Range queries are faster on B* trees. 
C  Btrees are for primary indexes and B* trees are for secondary indexes. 
D  The height of a B* tree is independent of the number of records. 
Question 30 
1 Read A
2 Read B
3 Write A
4 Read A
5 Write A
6 Write B
7 Read B
8 Write B
A  This schedule is serialized and can occur in a scheme using 2PL protocol 
B  This schedule is serializable but cannot occur in a scheme using 2PL protocol 
C  This schedule is not serialiable but can occur in a scheme using 2PL protocol 
D  This schedule is not seralisable and cannot occur in a scheme using 2PL protocol. 
Since cycle exist so not conflict serializable.
And we know that if the schedule is not serializable then it is not 2PL.
Hence correct option is (D).
Question 31 
Consider the schema R = (S T U V) and the dependencies S → T, T → U, U → V and V → S. Let R = (R1 and R2) be a decomposition such that R1 ∩ R2 ≠ ∅ . The decomposition is
A  not in 2NF 
B  in 2NF but not 3NF 
C  in 3NF but not in 2NF 
D  in both 2NF and 3NF 
And since every attribute is key so the decomposed relation will be in BCNF and hence in 3NF.
Question 32 
Consider the circuit shown below. In a certain steady state, the line Y is at '1'. What are the possible values of A, B and C in this state?
A  A = 0, B = 0, C = 1 
B  A = 0, B = 1, C = 1 
C  A = 1, B = 0, C = 1 
D  A = 1, B = 1, C = 1 
So the above equation is satisfied if either C=0 or A=0 and B=1.
Hence, Option (B) is correct.
Question 33 
Which of the following sets of component(s) is/are sufficient to implement any arbitrary Boolean function?
A  XOR gates, NOT gates 
B  2 to 1 multiplexors 
C  AND gates, XOR gates 
D  Threeinput gates that output (A⋅B) + C for the inputs A⋅B and C 
E  Both B and C 
(B) 2 to 1 multiplexors is functionally complete.
(C) XOR gate can be used to make a NOT gate. So, (AND, NOT) is functionally complete.
(D) With given gates and inputs NOT gate cannot be derived.
Hence, not complete.
Question 34 
Which of the following is/are correct?
A  An SQL query automatically eliminates duplicates 
B  An SQL query will not work if there are no indexes on the relations 
C  SQL permits attribute names to be repeated in the same relation 
D  None of the above 
→ If there are no indexes on the relation SQL, then also it works.
→ SQL does not permit 2 attributes to have same name in a relation.
Question 35 
Consider a Btree with degree m, that is, the number of children, c, of any internal node (except the root) is such that m ≤ c ≤ 2m1. Derive the maximum and minimum number of records in the leaf nodes for such a Btree with height h, h≥1. (Assume that the root of a tree is at height 0.)
A  Theory Explanation. 
Question 36 
Consider the set of relations
EMP(Employeeno, Deptno, Employeename, Salary) DEPT(Deptno, Deptname, Location)
Write an SQL query to:
(a) Find all employee names who work in departments located at "Calcutta" and whose salary is greater than Rs. 50,000.
(b) Calculate, for each department number, the number of employees with a salary greater than Rs. 100,000.
A  Theory Explanation. 
Question 37 
B^{+}trees are preferred to binary trees in databases because
A  Disk capacities are greater than memory capacities 
B  Disk access is much slower than memory access 
C  Disk data transfer rates are much less than memory data transfer rates 
D  Disks are more reliable than memory 
Question 38 
Given the relations
employee (name, salary, deptno) and department (deptno, deptname, address)
Which of the following queries cannot be expressed using the basic relational algebra operations (σ, π, ×, ⋈, ∪, ∩, )?
A  Department address of every employee 
B  Employees whose name is the same as their department name 
C  The sum of all employees’ salaries 
D  All employees of a given department 
Question 39 
Given the following relation instance.
x y z 1 4 2 1 5 3 1 6 3 3 2 2
Which of the following functional dependencies are satisfied by the instance?
A  XY → Z and Z → Y 
B  YZ → X and Y → Z 
C  YZ → X and X → Z 
D  XZ → Y and Y → X 
If for t1[A] = t2[A] then t1[Y] = t2[Y].
Question 40 
Given relations r(w, x) and s(y, z), the result of
select distinct w, x from r, s
is guaranteed to be same as r, provided
A  r has no duplicates and s is nonempty 
B  r and s have no duplicates 
C  s has no duplicates and r is nonempty 
D  r and s have the same number of tuples 
Question 41 
In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the following pairs is not equivalent?
A  x = 5 not AND (not (x = 5) 
B  x = 5 AND x > 4 and x < 6, where x is an integer 
C  x ≠ 5 AND not (x = 5) 
D  None of the above 
Question 42 
(a) Suppose you are given an empty B^{+}tree where each node (leaf and internal) can store up to 5 key values. Suppose values 1,2,….. 10 are inserted, in order, into the tree, Show the tree pictorially
(i) After 6 insertions, and
(ii) After all 10 insertions
Do NOT show intermediate stages.
(b) Suppose instead of splitting a node when it is full, we try to move a value to the left sibling. If there is no left sibling, or the left sibling is full, we split the node. Show the tree after values, 1, 2,….., 9 have been inserted. Assume, as in (a) that each node can hold up to 5 keys.
(c) In general, suppose a B^{+}tree node can hold a maximum of m keys, and you insert a long sequence of keys in increasing order. Then what approximately is the average number of keys in each leaf level node.
(i) In the normal case, and
(ii) With the insertion as in (b).
A  Theory Explanation is given below. 
(i)
(ii)
(b)
Question 43 
Consider a bank database with only one relation
transaction (transno, acctno, date, amount)
The amount attribute value is positive for deposits and negative for withdrawals.
(a) Define an SQL view TP containing the information.
(acctno, T1.date, T2.amount)
for every pair of transactions T1, T2 such that T1 and T2 are transaction on the same account and the date of T2 is ≤ the date of T1.
(b) Using only the above view TP, write a query to find for each account the minimum balance it ever reached (not including the 0 balance when the account is created). Assume there is at most one transaction per day on each account, and each account has had atleast one transaction since it was created. To simply your query, break it up into 2 steps by defining an intermediate view V.
A  Theory Explanation is given below. 
Question 44 
Consider a schema R(A,B,C,D) and functional dependencies A → B and C → D. Then the decomposition of R into R_{1}(AB) and R_{2}(CD) is
A  dependency preserving and lossless join 
B  lossless join but not dependency preserving 
C  dependency preserving but not lossless join 
D  not dependency preserving and not lossless join 
R_{1}∩R_{2} ≠ 0
Given R_{1}(A,B), R_{2}
R_{1}∩R_{2} = 0
Not lossless.
The given relation decomposed into R_{1}(A,B) and R_{2}(C,D) and there are only two functional dependencies A→B and C→D. So the given decomposition is dependency preserving.
Question 45 
Suppose the adjacency relation of vertices in a graph is represented in a table Adj(X,Y). Which of the following queries cannot be expressed by a relational algebra expression of constant length?
A  List of all vertices adjacent to a given vertex 
B  List all vertices which have self loops 
C  List all vertices which belong to cycles of less than three vertices 
D  List all vertices reachable from a given vertex 
(b) Finding a self loop is also simple (Oop(X,X))
(c) If a → b, b → c then c!=a, finding this is also simple.
(d) List all the elements reachable from a given vertex is too difficult in Relational Algebra.
Question 46 
Let r and s be two relations over the relation schemes R and S respectively, and let A be an attribute in R. Then the relational algebra expression σ_{A=a}(r⋈s) is always equal to
A  σ_{A=a} (r) 
B  r 
C  σ_{A=a} (r)⨝s 
D  None of the above 
(b) Display table
(c) A=a for all Tables r and s
Question 47 
R(A,B,C,D) is a relation. Which of the following does not have a lossless join, dependency preserving BCNF decomposition?
A  A → B, B → CD 
B  A → B, B → C, C → D 
C  AB → C, C → AD 
D  A → BCD 
Question 48 
Which of the following relational calculus expressions is not safe?
A  {t∃u ∈ R_{1} (t[A] = u[A])∧ ¬∃s ∈ R_{2} (t[A] = s[A])} 
B  {t∀u ∈ R_{1} (u[A]= "x" ⇒ ∃s ∈ R_{2} (t[A] = s[A] ∧ s[A] = u[A]))} 
C  {t¬(t ∈ R_{1})} 
D  {t∃u ∈ R_{1} (t[A] = u[A])∧ ∃s ∈ R_{2} (t[A] = s[A])} 
Question 49 
Consider a relation geq which represents “greater than or equal to”, that is, (x,y) ∈ geq only if y >= x.
create table geq ( Ib integer not null ub integer not null primary key 1b foreign key (ub) references geq on delete cascade )
Which of the following is possible if a tuple (x,y) is deleted?
A  A tuple (z,w) with z > y is deleted 
B  A tuple (z,w) with z > x is deleted 
C  A tuple (z,w) with w < x is deleted 
D  The deletion of (x,y) is prohibited 
Question 50 
Consider the relation examinee (regno, name, score), where regno is the primary key to score is a real number.
(a) Write a relational algebra using (∏,σ,ρ,×) to find the list of names which appear more than once in examinee.
(b) Write an SQL query to list the regno of examinees who have a score greater than the average score.
(c) Suppose the relation appears (regno, centr_code) specifies the center where an examinee appears. Write an SQL query to list the centr_code having an examinee of score greater than 80.
A  Theory Explanation is given below. 