SSC IMD SA 2017 – Question 188

The use of the ______ clause retains only one copy of duplicate tuples.

• Null
• Unique
• Not null
• Distinct

Detailed Explanation:

The correct answer is d) DISTINCT (अलग).

In standard SQL, query results generated by a SELECT statement may contain duplicate rows because relational algebra in SQL operates on multisets (bags) rather than pure sets by default:

• The DISTINCT clause is used in a SELECT statement to filter out duplicate rows from the query output, retaining only one unique copy of each tuple:

SELECT DISTINCT department_name FROM employees;

• How it works: The database engine sorts the resulting rows or uses a hash table to identify and discard duplicate occurrences of identical tuples.
• Contrasting other terms:
  • UNIQUE: An integrity constraint enforced on table columns to prevent duplicate entries during data insertion.
  • NULL / NOT NULL: Constraints defining whether a column accepts missing/unassigned values.