sql 语句查出的数据为空，怎么用个if语句判断，然后作出处理。

Question

select sum(price),userName
from food join diningcar on food.foodId=diningcar.foodId
join users on diningcar.userId=users.userId
where (comment=0 or comment=-1) and userName='zq'
group by userName
这条语句是查价格的总和，查出来的数据为空（这条语句是正确的，数据库里没有这个语句要查的相关信息）。我想做个判断。判断查出来的数据是否为空，如果为空，就给sum(price)赋值，使sum(price)=0.要保证数据库不发生改变，只是显示结果有变化。这能不能实现啊，大虾们帮帮忙

Answer 1

oracle：改为
select nvl(sum(price),0),userName
from food join diningcar on food.foodId=diningcar.foodId
join users on diningcar.userId=users.userId
where (comment=0 or comment=-1) and userName='zq'
group by userName
sqlserver改为
select isnull(sum(price),0),userName
from food join diningcar on food.foodId=diningcar.foodId
join users on diningcar.userId=users.userId
where (comment=0 or comment=-1) and userName='zq'
group by userName

Answer 2

可以实现，以sql server为例看：
if not exists(select userName from food join diningcar on food.foodId=diningcar.foodId join users on diningcar.userId=users.userId where (comment=0 or comment=-1) and userName='zq' group by userName)
select 0,'zq'
else
select sum(price),userName from food join diningcar on food.foodId=diningcar.foodId join users on diningcar.userId=users.userId where (comment=0 or comment=-1) and userName='zq' group by userName

方法二：
select isnull(sum(price),0),userName
from food join diningcar on food.foodId=diningcar.foodId
join users on diningcar.userId=users.userId
where (comment=0 or comment=-1) and userName='zq'
group by userName

不知道是不是你想要的结果，但是我有个疑问，你为什么不在程序里进行判断，而是要让sql语句判断呢？