ALTER TABLE slots
  ALTER COLUMN sf_display_type SET DEFAULT '1';

UPDATE slots
SET sf_display_type = '1'
WHERE sf_display_type IN ('', 'default', 'single')
  AND date_type = 'single';

UPDATE slots
SET sf_display_type = '2'
WHERE sf_display_type IN ('', 'default', 'range')
  AND date_type = 'range';