Page 349 - Informatics_Practices_Fliipbook_Class12
P. 349
18. What is meant by a NULL value? What are the reasons that lead to their occurrence in database relations?
Ans. The absence of a data item is represented by a special value called the null value. There are three situations
which may require the use of null values
(i) When a particular attribute does not apply to an entity,
(ii) Value of an attribute is unknown, although it exists;
(iii) When the value is unknown because it does not exist
19. Differentiate between the count(column) and count(*) function in SQL.
Ans. COUNT(column) counts non-null values in the specified column, while COUNT(*) counts all rows, regardless of
whether they contain NULL values or not.
20. How will you remove records having no products from the table "Suppliers" having fields Sno, name, area,
products?
Ans. DELETE FROM Suppliers WHERE products = NULL;
21. Your friend Ranjana complaints that somebody has created a fake profile on Facebook and defaming her
character with abusive comments and pictures.
Identify the type of cybercrime for these situations.
Ans. Cyber Bullying
22. Name the clause of SQL used to arrange the rows in ascending order based on an attribute.
Ans. ORDER BY
23. List the aggregate functions used in SQL.
Ans. SUM, AVG, COUNT, MAX, and MIN
24. What is primary key?
Ans. Primary key is a set of one or more attributes that uniquely identifies a tuple in a relation.
25. What is foreign key?
Ans. An attribute or a set of attributes in one table that refer to the primary key of another table is known as a foreign
key.
26. What is MySQL?
Ans. MySQL is an open source RDBMS that relies on SQL for processing the data in the database. The database is
available for free under the terms of General Public License (GPL).
27. Define the term topology. What are the popular network topologies?
Ans. Topology is the pattern of interconnection of nodes in a local area network (LAN). Popular network topologies
include Mesh, Ring, Bus, Star and Tree.
28. What do you mean by a modem? Why is it used?
Ans. A modem (Modulator - Demodulator) is a peripheral device that enables a computer to transmit data over,
telephone or cable lines.
It combines the modulation and demodulation functions by transforming a digital signal into an analog signal
and vice versa. Digital data to be sent from a computer is modulated for transmission over telephone lines.
Similarly, at the receiving end, analog data is demodulated into digital form.
Viva Voce Questions 335

