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. UNIQUE
B. FOREIGN KEY
C. PRIMARY KEY
D. CHECK
Answer: Option A
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:
ACID stands for Atomicity, Consistency, Isolation, and Durability. Durability ensures that once a transaction commits, its changes survive system failures.
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