SELECT tt.form_name,
       100.00*tt.part_form/tt.total_forms as percent_forms
FROM (
  SELECT 
      t.form_name,
      t.cn part_form,
      SUM(t.cn) OVER() total_forms
  FROM (
    SELECT
            SUM((
              SELECT COUNT(8) FROM drug_exposure WHERE drug_concept_id = drug.concept_id)
            ) cn,
            drugform.concept_name            form_name
      FROM  vocabulary.concept               ingredient,
            vocabulary.concept_ancestor      a,
            vocabulary.concept               drug,
            vocabulary.concept_relationship  r,
            vocabulary.concept               drugform
      WHERE ingredient.concept_id    =  1125315 -- Acetaminophen
      AND   ingredient.concept_class = 'Ingredient'
      AND   ingredient.concept_id    = a.ancestor_concept_id
      AND   a.descendant_concept_id  = drug.concept_id
      AND   drug.concept_level       = 1 -- ensure it is drug product
      AND   drug.concept_id          = r.concept_id_1
      AND   r.concept_id_2           = drugform.concept_id
      AND   drugform.concept_class   = 'Dose Form'
      GROUP BY
              drugform.concept_name
  ) t  
  WHERE t.cn>0 -- don't count forms that exist but are not used in the data
) tt
WHERE tt.total_forms > 0 -- avoid division by 0
ORDER BY perc_forms desc
;