Please always say what version you are using. In 2005 use CUBE/ROLLUP. In 2008 use GROUPING SETS.

SELECT JobNo, PartNo, SUM(Quantity) Quantity
FROM Stock
GROUP BY PartNo,JobNo WITH ROLLUP;

SELECT JobNo, PartNo, SUM(Quantity) Quantity
FROM Stock
GROUP BY GROUPING SETS ((JobNo,PartNo),(PartNo),());