ALTER TABLE parts_inventory
  ADD COLUMN hsn_code VARCHAR(8) DEFAULT NULL;

ALTER TABLE invoice_lines
  ADD COLUMN hsn_code VARCHAR(8) DEFAULT NULL;

ALTER TABLE workshop_settings
  ADD COLUMN parts_hsn_code VARCHAR(8) DEFAULT '8708';

UPDATE workshop_settings
SET parts_hsn_code = COALESCE(NULLIF(parts_hsn_code, ''), '8708')
WHERE id = 1;