ALTER TABLE prestations
  ADD COLUMN IF NOT EXISTS genre_groupe TEXT,
  ADD COLUMN IF NOT EXISTS composition_public TEXT;

UPDATE prestations
SET
  genre_groupe = COALESCE(NULLIF(TRIM(genre_groupe), ''), 'mixte'),
  composition_public = COALESCE(NULLIF(TRIM(composition_public), ''), 'adulte')
WHERE genre_groupe IS NULL
   OR TRIM(genre_groupe) = ''
   OR composition_public IS NULL
   OR TRIM(composition_public) = '';