USE lifanghe_education;

CREATE TABLE IF NOT EXISTS appointment_duplicates_backup LIKE appointments;

INSERT INTO appointment_duplicates_backup
SELECT a.*
FROM appointments a
JOIN appointments b
  ON a.user_id = b.user_id
 AND (a.created_at < b.created_at OR (a.created_at = b.created_at AND a.id < b.id))
LEFT JOIN appointment_duplicates_backup bak ON bak.id = a.id
WHERE bak.id IS NULL;

DELETE a
FROM appointments a
JOIN appointments b
  ON a.user_id = b.user_id
 AND (a.created_at < b.created_at OR (a.created_at = b.created_at AND a.id < b.id));

SET @schema_name = DATABASE();
SET @has_unique := (
  SELECT COUNT(1)
  FROM INFORMATION_SCHEMA.STATISTICS
  WHERE TABLE_SCHEMA = @schema_name
    AND TABLE_NAME = 'appointments'
    AND INDEX_NAME = 'uk_appointments_user'
);
SET @add_unique_sql := IF(
  @has_unique = 0,
  'ALTER TABLE appointments ADD UNIQUE KEY uk_appointments_user (user_id)',
  'SELECT 1'
);
PREPARE add_unique_stmt FROM @add_unique_sql;
EXECUTE add_unique_stmt;
DEALLOCATE PREPARE add_unique_stmt;