You tried to add a UNIQUE constraint on column username but got an error. What is the most likely cause?

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SQL - Table Constraints

You tried to add a UNIQUE constraint on column username but got an error. What is the most likely cause?

AThe column <code>username</code> is not indexed

BThe column <code>username</code> contains duplicate values already

CUNIQUE constraints cannot be added after table creation

DThe table does not exist

Step-by-Step Solution

Solution:

Step 1: Understand UNIQUE addition failure and evaluate causes
Adding a UNIQUE constraint fails if existing data violates uniqueness (duplicates exist). "The column username contains duplicate values already" identifies the cause. A non-existent table is unrelated (assuming it exists); UNIQUE can be added later; indexing is irrelevant or automatic.
Final Answer:
The column username contains duplicate values already -> Option B
Quick Check:
Duplicates prevent adding UNIQUE constraint [OK]

Quick Trick: Check for duplicates before adding UNIQUE [OK]

Common Mistakes:

MISTAKES

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