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4

sql

-- Show users who selected any course
SELECT name
FROM users u
WHERE EXISTS (
  SELECT 1
  FROM user_courses uc
  WHERE uc.user_id = u.user_id
);

-- Show users who have not selected any course
SELECT name
FROM users u
WHERE NOT EXISTS (
SELECT 1
FROM user_courses uc
WHERE uc.user_id = u.user_id
);

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sql