pgtips3.sql

 similarities as (
    select distinct
    ss.user_id as id,
    sum((sr.symptom_severity - coalesce(target.symptom_severity,0)) ^ 2) OVER (PARTITION BY ss.user_id),
    count(sr.id) OVER (PARTITION BY ss.user_id) as count

    from latest_symptom_surveys ss
      join symptom_reports sr on sr.symptom_survey_id = ss.id
      left join target on target.symptom_id = sr.symptom_id
      join users u on u.id = ss.user_id
    where u.disease_id = #{self.disease_id}
      and u.id <> #{self.id})