Question

In a relational database, a table has 5 attributes and 1000 records. If a query returns 200 records after applying a condition, what is the selectivity of the query?

• A. 0.2
• B. 0.5
• C. 0.8
• D. 1.0

Hint:

Low selectivity means a query filters out most records, improving efficiency.

Answer 1

The Correct Option is A

Step 1: Define selectivity
Selectivity = Fraction of records retrieved by a query.
Formula: Selectivity = $\frac{\text{Number of records returned}}{\text{Total records}}$.
Step 2: Substitute values
Records returned = 200, Total records = 1000.
Selectivity = $\frac{200}{1000}$.
Step 3: Calculate
$\Rightarrow$ Selectivity = 0.2.