-》完整的select语句及执行顺序（必须记住）
5...select 5.2->distinct 7...top n [percent] 5.1->列名 聚合函数(1.2->列名或表达式)
1...1.1->from 表名
2...where 条件
3...group by ... 4...having ...
6...order by ...
-》实际练习
select distinct top 1 tsclassid,COUNT(*) AS 人数,avg(tsage) as 平均年龄
from tblstudent
where tsGender='男'
group by tsclassid having tsclassid>3
order by 平均年龄 desc
-》问执行顺序是如何？
1、from tblstudent
2、where tsGender='男'
3、group by tsclassid
4、having tsclassid>3
5、select
5.1 tsclassid,COUNT(*) AS 人数,avg(tsage) as 平均年龄
5.2 distinct
6、order by 平均年龄 desc
7、top 1
inner join(等值连接) 只返回两个表中联结字段相等的行
left join(左联接) 返回包括左表中的所有记录和右表中联结字段相等的记录
right join(右联接) 返回包括右表中的所有记录和左表中联结字段相等的记录