This is the DBMS questions and answers section on "Aggregate Functions" with explanation for various interview, competitive examination and entrance test. Solved examples with detailed answer description, explanation are given and it would be easy to understand.
1. Aggregate functions are functions that take a ___________ as input and return a single value.
- A. Collection of values
- B. Single value
- C. Aggregate value
- D. Both a & b
Answer: Option A
Explanation:
Collection of values
2. Select __________ from instructor where dept name= ’Comp. Sci.’;
Which of the following should be used to find the mean of the salary ?
- A. Mean(salary)
- B. Avg(salary)
- C. Sum(salary)
- D. Count(salary)
Answer: Option B
Explanation:
Avg(salary)
Avg() is used to find the mean of the values.
3. All aggregate functions except _____ ignore null values in their input collection.
- A. Count(attribute)
- B. Count(*)
- C. Avg
- D. Sum
Answer: Option B
Explanation:
Count(*)
* is used to select all values including null.
4. A Boolean data type that can take values true, false, and________ .
Answer: Option D
Explanation:
Unknown
Unknown values do not take null value but it is not known.
5. Select count (____ ID)
from teaches
where semester = ’Spring’ and year = 2010;
If we do want to eliminate duplicates, we use the keyword ______in the aggregate expression.
- A. Distinct
- B. Count
- C. Avg
- D. Primary key
Answer: Option A
Explanation:
Distinct
Distinct keyword is used to select only unique items from the relation.
