主键的值可以为空么？

can the value of primary key be NULL? no

答案 ： 不行

创建索引是为了提高性能么？

Is the index used to improve the peformance? 答案：yes

答案： 是

关键字 select distinct 是用来分组的么？

is the keyword "select distinct" used to group result-set by one or more columns? no

答案： 不是

哪个关键字是用来对结果排序？

which keyword is used to sort the result-set? order by

答案 ： Order by

哪个关键字是用来修改已经存在的数据？

which keyword is used to modify the existing records in table? update

答案： update