SQL求和的语句，为什么是错的

[Netcean]

select t.taocanName,o.taocanCount From [order] as o,taocan as t where o.loupanID=3 And o.taocanID=t.ID
这个语句执行的效果如下图，现在想把相同taocanName的数量加起来，如

牛肉煎蛋饭 6
干烧对虾 6
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select t.taocanName,o.taocanCount From [order] as o,taocan as t where o.loupanID=3 And o.taocanID=t.ID group by o.taocanCount,t,taocanName
如果换成这语句，taocanCount总数是错误的

[No_Data_Found]

select t.taocanName,sum(o.taocanCount) From [order] as o,taocan as t where o.loupanID=3 And o.taocanID=t.ID group by t.taocanName 

[Netcean]

谢谢，正确的结果。