select code,sum(sumber) as num from (
select code,number from aaa where mark='出货'
union all
select code,-number as number from aaa where mark='退货')
group by code
sql server
SELECT a.code, SUM(a.number - b.number) AS SUM
FROM T a INNER JOIN
t b ON a.code = b.code
WHERE a.mark = '进货' AND b.mark = '退货'
GROUP BY a.code
SELECT a.code, (a.suma-b.sumb) AS [number]
FROM [SELECT aaa.code, Sum(aaa.number) AS suma
FROM aaa
WHERE Mark='出货'
GROUP BY aaa.code]. AS a LEFT JOIN [select aaa.code,sum(aaa.number) as sumb
from aaa
where Mark='退货'
GROUP by aaa.code]. AS b ON a.code = b.code;