我有这些数据：
name qty date flag --------------------------------------------- abc 255 11/10/1986 12:00:00 AM IN abc 300 11/10/2010 12:00:00 AM IN abc 12 11/10/2012 12:00:00 AM OUT abc 13 11/9/2010 12:00:00 AM OUT NULL NULL NULL NULL
解决方法
SELECT SUM(case flag when 'IN' then qty else -qty end) from table WHERE ..... Your conditions here ...
