（转载）http://hi.baidu.com/lylegend13/item/a79f17eb51f5dff7e0a5d43b

1.

select count(distinct CName) from Course

2.

select count(CName) from (select distinct CName from Course) as temp

as可有可无。temp作为别名，若无则提示错误：1248 - Every derived table must have its own alias