Which of the following is the correct syntax to add a UNIQUE constraint to an existing column email in a table users?

easy📝 Syntax Q12 of 15

SQL - Table Constraints

Which of the following is the correct syntax to add a UNIQUE constraint to an existing column email in a table users?

AALTER TABLE users ADD UNIQUE email;

BALTER TABLE users ADD CONSTRAINT UNIQUE email;

CALTER TABLE users ADD UNIQUE (email);

DALTER TABLE users ADD UNIQUE CONSTRAINT (email);

Step-by-Step Solution

Solution:

Step 1: Recall ALTER TABLE syntax for UNIQUE and check options
The correct syntax to add a UNIQUE constraint is: ALTER TABLE table_name ADD UNIQUE (column_name); "ALTER TABLE users ADD UNIQUE (email);" matches exactly. The other options misuse keywords or omit parentheses.
Final Answer:
ALTER TABLE users ADD UNIQUE (email); -> Option C
Quick Check:
ALTER TABLE ADD UNIQUE (column) syntax = ALTER TABLE users ADD UNIQUE (email); [OK]

Quick Trick: Use ADD UNIQUE (column) with parentheses [OK]

Common Mistakes:

MISTAKES

Master "Table Constraints" in SQL

9 interactive learning modes - each teaches the same concept differently

Want More Practice?

15+ quiz questions · All difficulty levels · Free

More SQL Quizzes