小SQL,大学问

下面的sql语句有错误吗？

select item.fchrItemCode,item.fchrItemName,item.glotSafeNum,itemstocks.flotQuantity,
(item.glotSafeNum-itemstocks.flotQuantity) as differ
from item inner join itemstocks on item.fchrItemID=itemstocks.fchrItemID
where differ>0.0 order by differ desc

希望你答对了，是有误的，“列名 'differ' 无效。”

错误的主要原因是没有搞清楚select语句的运行过程，select语句先执行from子句，接着执行where子句，最后才是select和order by。

所以正确的sql语句应该是：

select item.fchrItemCode,item.fchrItemName,item.glotSafeNum,itemstocks.flotQuantity,
(item.glotSafeNum-itemstocks.flotQuantity) as differ
from item inner join itemstocks on item.fchrItemID=itemstocks.fchrItemID
where (item.glotSafeNum-itemstocks.flotQuantity)>0.0 order by differ desc