Monday, March 28, 2022

ISRO Questions - Database Management Systems

Recruitment Entrance Test for Scientist/Engineer
Previously asked questions on Database Management Systems

ISRO 2007
1. Which commands are used to control access over objects in relational database?
(d) None of these

Ans: option(b)
GRANT, REVOKE, and DENY are the commands which are used to control access over objects in relational data. We user the term Data Control Language (DCL) to represent these commands. 
CREATE, ALTER, DROP, TRUNCATE comes under Data Definition Language (DDL) used for defining database schemas.
CASCADE is used in conjunction with ON DELETE or ON UPDATE. It means that the child data is either deleted or updated when the parent data is deleted or updated.
QUIST (Query lmprovement through Semantic Transformation) is a system for semantic query optimization in relational databases.
MVD means multivalued dependency - Refer:

ISRO 2007
2. Which of the following is aggregate function in SQL?
(a) Avg
(b) Select
(c) Ordered by
(d) distinct

Ans: option(a)
An aggregate function allows you to perform a calculation on a set of values to return a single scalar value. Most commonly used aggregate functions are:
AVG – calculates the average of a set of values.
COUNT – counts rows in a specified table or view.
MIN – gets the minimum value in a set of values.
MAX – gets the maximum value in a set of values.
SUM – calculates the sum of values.

ISRO 2007
3. A view of database that appears to an application program is known as
(a) Schema
(b) Subschema
(c) Virtual table
(d) None of these

Ans: option (b)
Database schema is the design or structure or skeleton of a database. There exist schema at each level of DBMS architecture. At internal level, there exist one schema known as Physical schema, which describes how data is actually stored in memory. This is of low abstraction and it is not visible. At logical level, there exists Logical Schema which describes the structure of database and relationships in the database. Visible to or used by data base administrators or programmers.
At external level we have Sub-schema which describes the actual view of data seen by end users. This level hides many irrelevant data to users.

ISRO 2007
4. Which operation is used to extract specified columns from a table?
(a) Project
(b) Join
(c) Extract
(d) Substitute

Ans: option (a)
To extract specific rows or tuples from a table that satisfy a condition we use Select operation. Select operation is denoted by Greek letter sigma(σ).
For example:
The above operation specifies the action of finding all tuples from Products table with price greater than 1000.

If we want to select the values of a few attributes (or fields), rather than selecting all attributes of the Table (Relation) we use project operation. Project operation is denoted by uppercase Greek letter pi().
For example:
The above query lists all product name and price. Note: Project operation produces a relation and since relation is a set, any duplicate rows are eliminated.

ISRO 2007
5. BCNF is not used for cases where a relation has
(a) Two (or more) candidate keys
(b) Two candidate keys and composite
(c) The candidate key overlap
(d) Two mutually exclusive foreign keys

Ans: option(d)

A candidate key is a column/attribute, or set of columns/attributes, in a table that can uniquely identify any database record/row/tuple. Each table may have one or more candidate keys.
Composite Key: When a key is composed of more than one column, it is known as a composite key.
Overlapping candidate keys are composite candidate keys with at least one attribute/column in common. For example if X,Y is a candidate key and X,Z is another candidate key then we can say that X,Y and X,Z are overlapping candidate keys because they share X attribute in common.

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