Question

Let us consider two relations HISTORY and SCIENCE as depicted in the following Tables 1 and 2 respectively: 

Table 1: HISTORY

SNo	NAME	CLASS
1	MOHAN	6A
2	SANJAY	6B
3	YOGESH	7A

Table 2: SCIENCE

SNo	NAME	CLASS
1	NARESH	6B
2	SANJAY	6B
3	YOGESH	7A

 
What is the output of HISTORY - SCIENCE?

• A.

  SNo	NAME	CLASS
  1	MOHAN	6A

• B.

  SNo	NAME	CLASS
  1	MOHAN	6A
  2	SANJAY	6B
  3	YOGESH	7A

• C.

  SNo	NAME	CLASS
  1	NARESH	6B
  2	SANJAY	6B
  3	YOGESH	7A

• D.

  SNo	NAME	CLASS
  1	NARESH	6B

Hint:

Set difference in databases means: “what’s in the first table but not in the second.” Watch for exact matches!

Answer 1

The Correct Option is A

To find the result of the set difference operation HISTORY - SCIENCE, follow these steps:
1. The set difference operation will return rows from the HISTORY table that are not present in the SCIENCE table.
2. Compare each row from HISTORY with every row in SCIENCE.
3. Identify rows in HISTORY with no matching row in SCIENCE based on 'NAME' and 'CLASS'.

The rows in HISTORY are:
- (1, MOHAN, 6A)
- (2, SANJAY, 6B)
- (3, YOGESH, 7A)

The rows in SCIENCE are:
- (1, NARESH, 6B)
- (2, SANJAY, 6B)
- (3, YOGESH, 7A)

By comparing,
- (1, MOHAN, 6A) is present in HISTORY but not in SCIENCE.
- (2, SANJAY, 6B) is present in both tables.
- (3, YOGESH, 7A) is present in both tables.

Thus, the only row not present in SCIENCE is:

SNo	NAME	CLASS
1	MOHAN	6A