UPDATE invoices SET
id = CONCAT (client,'-',
             order,
             IFNULL(variant,''),
             IF(subinvoice IS NULL, '',CONCAT('-',subinvoice))
             );

DELIMITER //
DROP TRIGGER IF EXISTS invoice_generate_id//
CREATE TRIGGER invoice_generate_id
BEFORE INSERT ON invoices
FOR EACH ROW
BEGIN
  NEW.id = CONCAT (client,'-',
             order,
             IF (variant IS NOT NULL) THEN
                   variant
             ELSE ''
             END IF;
             ,
             IF (subinvoice IS NOT NULL) THEN
              CONCAT('-',subinvoice))
             ELSE ''
             END IF;
             );
END//
DELIMITER ;