[Solved] You have this SQL statement to add a UNIQUE constraint:ALTER TABLE users ADD UNIQUE (username);But it fails with an error. What is the most likely cause? | SQL

SQL - Table Constraints

You have this SQL statement to add a UNIQUE constraint:

ALTER TABLE users ADD UNIQUE (username);

But it fails with an error. What is the most likely cause?

AThe username column contains duplicate values already.

BThe UNIQUE keyword is misspelled.

CThe table users does not exist.

DUNIQUE constraints cannot be added after table creation.

Step-by-Step Solution

Solution:

Step 1: Understand UNIQUE constraint requirements
Adding a UNIQUE constraint requires all existing values in the column to be unique.
Step 2: Identify why ALTER TABLE fails
If duplicates exist in username, the database rejects the constraint addition with an error.
Final Answer:
The username column contains duplicate values already. -> Option A
Quick Check:
Existing duplicates block UNIQUE constraint [OK]

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

Common Mistakes:

MISTAKES

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