[Solved] Why might a UNIQUE constraint be preferred over a PRIMARY KEY constraint on a column? — Ans: UNIQUE allows multiple NULLs, PRIMARY KEY does not | SQL

SQL - Table Constraints

Why might a UNIQUE constraint be preferred over a PRIMARY KEY constraint on a column?

AUNIQUE constraints cannot be dropped, PRIMARY KEY can

BUNIQUE automatically creates an index, PRIMARY KEY does not

CPRIMARY KEY allows duplicates, UNIQUE does not

DUNIQUE allows multiple NULLs, PRIMARY KEY does not

Step-by-Step Solution

Solution:

Step 1: Compare UNIQUE and PRIMARY KEY constraints
PRIMARY KEY requires unique and NOT NULL values; UNIQUE enforces uniqueness on non-NULL values and permits multiple NULLs.
Step 2: Identify key difference
UNIQUE allows multiple NULLs, which PRIMARY KEY does not.
Final Answer:
UNIQUE allows multiple NULLs, PRIMARY KEY does not -> Option D
Quick Check:
UNIQUE permits NULLs, PRIMARY KEY forbids them [OK]

Quick Trick: UNIQUE allows NULLs; PRIMARY KEY requires NOT NULL [OK]

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Why might a UNIQUE constraint be preferred over a PRIMARY KEY constraint on a column?