Insert User with Unique Phone

In a single SQL query, insert a new user with a phone number and make sure phone number is unique.

Asked in the Technical Round 1 stage. Reported follow-ups included the benefits of UNIQUE and which data structure implements indexing.

Output

1. Return user_id, full_name, and phone_number for the inserted user.
2. Insert the user only if the phone number is not already present.
3. Return at most one row, with no required ordering.