DBMS gate questions

In this tutorial some dbms gate questions are explained.These dbms gate questions can be practice for gate exam preparation. These questions are asked in previous year GATE Exam. This tutorial covers the gate questions for dbms. This Tutorial also  covers the questions based on sql queries for interviews.

Q1. Read the following statement and find the correct option.

P :  A query in Structural Query Language  can contain HAVING clause without having GROUP BY clause

Q : A  query in Structural Query Language can contain a HAVING clause only when GROUP  BY clause is also there in query.

R : All attributes used in the GROUP BY clause must appear in the SELECT clause

S : No It is not necessary that all attributes used in the GROUP BY clause need to appear in the SELECT clause

a.            P and R

b.            P and S

c.             Q and R

d.            Q and S

Concept : As per the SQL concepts option c is right but suppose if tak about execution of these sql query on different database . Let us take MYSQL then option B will also be right. What happend if  

If we use a HAVING clause without a GROUP BY clause in this situation  the HAVING condition applies to all rows that satisfy the search condition and in the result all rows that satisfy the search condition make up a single group. 

Q2. A database table  named as Loan_Records is given below.

Borrower    Bank_Manager   Loan_Amount

 Ramesh      Sunderajan     10000.00

 Suresh      Ramgopal       5000.00

 Mahesh      Sunderajan     7000.00

What is the output of the following SQL query?

SELECT Count(*)

FROM  ( (SELECT Borrower, Bank_Manager

       FROM   Loan_Records) AS S

        NATURAL JOIN (SELECT Bank_Manager,

                             Loan_Amount

                      FROM   Loan_Records) AS T );

a.            3

b.            9

c.             5

d.            6

Answer: C

Concept : Result of first subquery will be Table S having the records –

Borrower    Bank_Manager
————————–
Ramesh      Sunderajan
Suresh      Ramgqpal
Mahesh      Sunderjan

Result of Second sub query will be table T having the records-

Bank_Manager   Loan_Amount
—————————
Sunderajan      10000.00
Ramgopal        5000.00
Sunderjan       7000.00

When Natural Join is performed on these two table S and  T the will After Join resultant table will have the following records

Borrower  Bank_Manager   Load_Amount
————————————
Ramesh    Sunderajan     10000.00
Ramesh    Sunderajan     7000.00
Suresh    Ramgopal       5000.00
Mahesh    Sunderajan     10000.00
Mahesh    Sunderajan     7000.00

Here point ot remember is that in natural join matching occur  on column name with same name

Q.3. Suppose a table T has two
columns X and Y. Type of each column is integer. Once the table is created then
a record X=1, Y=1 is inserted in the table. If MX and My represent  the respective maximum values of X and Y among
all records in the table at any instant of time. Using MX and MY, some  new data is inserted in the table 128 times
with X and Y values being MX+1, 2*MY+1 respectively. It must be remember that
each time  values of MX and MY change
after insertion. If following SQL query after the steps mentioned above is
carried out then what will be the output?

SELECT Y FROM T WHERE X=7;

a.            127

b.            255

c.             129

d.            257

In order to understand the logic see Question no 30 for gate (cs/it) 2011 paper with solution here

Q4. Given the following schema:

     employees(emp-id, first-name, last-name, hire-date, dept-id, salary)

     departments(dept-id, dept-name, manager-id, location-id)

Which of following query should be execute to display the last names and hire dates of all latest hires in their respective departments in the location ID 1700. You issue the:

SELECT last-name, hire-date

     FROM employees

     WHERE (dept-id, hire-date) IN

     (SELECT dept-id, MAX(hire-date)

     FROM employees JOIN departments USING(dept-id)

     WHERE location-id = 1700

     GROUP BY dept-id);

What is the output?

a.            It executes but does not give the correct result

b.            It executes and gives the correct result

c.             generates an error because of pairwise comparison

d.            It generates an error because the GROUP BY clause cannot be used with table joins in a subquery.

Answer B

Concept: This query will generate the correct result because at first the inner query will return the last max hire date in every department located at id 1700. Then the outer query take the result of inner query and provide the correct result.

Q5.Consider the following  relational schema for a train reservation database . Passenger (pid, pname, age) Reservation (pid, class, tid)

Table: Passenger

pid   pname   age

—————–

 0    Sachin   65

 1    Rahul    66

 2    Sourav   67

 3    Anil     69

Table : Reservation

pid  class  tid

—————

 0    AC   8200

 1    AC   8201

 2    SC   8201

 5    AC   8203

 1    SC   8204

 3    AC   8202

What  are the pids returned by the following SQL query for the above instance of the tables?

 SELECT pid

FROM Reservation ,

WHERE class ‘AC’ AND

    EXISTS (SELECT *

       FROM Passenger

       WHERE age > 65 AND

       Passenger. pid = Reservation.pid)

a.            1, 0

b.            1, 2

c.             1, 3

d.            1, 5

Answer : C

Concept : The inner query SELECT *

       Passenger. pid = Reservation.pid) will return 4 tuples

1 AC 8201
2 SC 8201
1 SC 8204

3 AC 8202

Now the outer query  will select those tuple only which have the class AC , as a result Pid return will be 1 and 3 so Option C is correct.

Q6. A relational schema is given below:

Suppliers(sid:integer, sname:string, city:string, street:string)

Parts(pid:integer, pname:string, color:string)

Catalog(sid:integer, pid:integer, cost:real)

If the following sql  query is executed for the  above database:

SELECT S.sname

    FROM Suppliers S

        WHERE S.sid NOT IN (SELECT C.sid

                            FROM Catalog C

                            WHERE C.pid NOT IN (SELECT P.pid 

                                                FROM Parts P

                                                WHERE P.color<> ‘blue’))

Suppose some records are inserted in the table. Then find out that which one of the following is the correct interpretation of the above query?

a.            Find the names of all suppliers who have supplied a non-blue part.

b.            Find the names of all suppliers who have not supplied a non-blue part.

c.             Find the names of all suppliers who have supplied only blue parts.

d.            Find the names of all suppliers who have not supplied only blue parts.

Q7. Take a table employee(empId, name, department, salary) and the two queries Q1 ,Q2 as given  below. Suppose that department  5 has more than one employee and we want to find the employees who get higher salary than anyone in the department 5, which one of the statements is TRUE for any arbitrary employee table?

Q1 : Select e.empId

     From employee e

     Where not exists

        (Select * From employee s where s.department = “5” and

                                        s.salary >=e.salary)

Q2 : Select e.empId

     From employee e

     Where e.salary > Any

    (Select distinct salary From employee s Where s.department = “5”)

a.            Q1 is the correct query

b.            Q2 is the correct query

c.             Both Q1 and Q2 produce the same answer

d.            Neither Q1 nor Q2 is the correct query

Q8. For the following two statements

    S1: Declaration of a foreign key can always be replaced by an equivalent check assertion in SQL.

    S2: Given the table R(a,b,c) where a and b together form the primary key, the  following is a valid table definition.

        CREATE TABLE S (     a INTEGER, d INTEGER, e INTEGER,    PRIMARY KEY (d),  FOREIGN KEY (a) references R)

The find the correct option among the following.

a.            S1 is TRUE and S2 is FALSE

b.            Both S1 and S2 are TRUE

c.             S1 is FALSE and S2 is TRUE

d.            Both S1 and S2 are FALSE

Q9. SQL allows tuples in relations  and correspondingly defines the multiplicity of tuples in the result of joins. Which one of the following queries always gives the same answer as the nested query shown below.

    select * from R where a in (select S.a from S)

a.            select R.* from R, S where R.a=S.a (D)

b.            select distinct R.* from R,S where R.a=S.a

c.             select R.* from R,(select distinct a from S) as S1 where R.a=S1.a

d.            select R.* from R,S where R.a=S.a and is unique R

 Q10. There are three table  above tables A, B and C as given below

 Table A

Id   Name    Age

—————-

12   Arun    60

15   Shreya  24

99   Rohit   11

Table B

Id   Name   Age

—————-

15   Shreya  24

25   Hari    40

98   Rohit   20

99   Rohit   1

Table C

Id   Phone  Area

—————–

10   2200   02

99   2100   01

 Then determine the number of tuples  return in the result of the following SQL query.

SELECT A.id

FROM   A

WHERE  A.age > ALL (SELECT B.age   FROM   B

                    WHERE  B. name = “arun”);

a.            4

b.            3

c.             0

d.            1

Answer: B

Concept: Value of A.age should be greater than all the values return by the execution of subquery. When we look at table B then we found that there is no name as arun which is mentioned in the subquery. So this subquery will return NULL. So in this case condition become true for all the rows of table A. Hence correct answer is 3.

More questions will be updated soon….!