Which constraint ensures that every value in a specific column is unique across all rows in that table, but still permits a NULL value? MCQ with Answer and Explanation
Which constraint ensures that every value in a specific column is unique across all rows in that table, but still permits a NULL value?
A. CHECK
B. PRIMARY KEY
C. FOREIGN KEY
D. UNIQUE
Answer: Option D
Solution (By JKSSB Mock Tests)
The UNIQUE constraint prevents duplicate entries in a column. Unlike a Primary Key, a UNIQUE constraint typically allows one or more rows to store NULL values depending on the database engine.
Explanation:
Physical Data Independence is the ability to modify the physical schema (like storage devices or file structures) without changing the conceptual or external schemas.
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