[Solved] How do you correctly add a UNIQUE constraint to the existing column user_id in the table members? — Ans: ALTER TABLE members ADD CONSTRAINT unique_user_id UNIQUE (user_id); | SQL

SQL - Table Constraints

How do you correctly add a UNIQUE constraint to the existing column user_id in the table members?

AALTER TABLE members ADD CONSTRAINT unique_user_id UNIQUE (user_id);

BALTER TABLE members ADD UNIQUE user_id;

CALTER TABLE members MODIFY user_id UNIQUE;

DALTER TABLE members SET UNIQUE user_id;

Step-by-Step Solution

Solution:

Step 1: Syntax for adding UNIQUE constraint
To add a UNIQUE constraint to an existing column, use ALTER TABLE with ADD CONSTRAINT and specify UNIQUE on the column.
Step 2: Evaluate options
ALTER TABLE members ADD CONSTRAINT unique_user_id UNIQUE (user_id); uses correct syntax. ALTER TABLE members ADD UNIQUE user_id; is invalid syntax. ALTER TABLE members MODIFY user_id UNIQUE; is incorrect because MODIFY does not add constraints. ALTER TABLE members SET UNIQUE user_id; is not valid SQL.
Final Answer:
ALTER TABLE members ADD CONSTRAINT unique_user_id UNIQUE (user_id); -> Option A
Quick Check:
ALTER TABLE ADD CONSTRAINT UNIQUE (column) [OK]

Quick Trick: Use ADD CONSTRAINT with UNIQUE for existing columns [OK]

Common Mistakes:

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How do you correctly add a UNIQUE constraint to the existing column user_id in the table members?