Which of the following is the correct syntax to add a PRIMARY KEY constraint to an existing table named users on column user_id?

easy📝 Syntax Q3 of 15

SQL - Table Constraints

Which of the following is the correct syntax to add a PRIMARY KEY constraint to an existing table named users on column user_id?

AALTER TABLE users SET PRIMARY KEY user_id;

BALTER TABLE users ADD PRIMARY KEY (user_id);

CALTER TABLE users ADD CONSTRAINT PRIMARY KEY user_id;

DALTER TABLE users ADD PRIMARY KEY user_id;

Step-by-Step Solution

Solution:

Step 1: Recall correct ALTER TABLE syntax for PRIMARY KEY
The correct syntax is ALTER TABLE table_name ADD PRIMARY KEY (column_name); with parentheses around the column.
Step 2: Compare options
Only ALTER TABLE users ADD PRIMARY KEY (user_id); uses correct syntax with ADD PRIMARY KEY and parentheses.
Final Answer:
ALTER TABLE users ADD PRIMARY KEY (user_id); -> Option B
Quick Check:
Correct syntax for PRIMARY KEY addition = ALTER TABLE users ADD PRIMARY KEY (user_id); [OK]

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

Common Mistakes:

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