概念了解：
select
语句中包含一个查询子句
（五子句，包括
where
、
group by
、
order by
、
having
和
limit
）
1.where
经常用到，就不细说了，可以搭配一些比较运算符>,<…；逻辑运算符&&，||，!
2.group by
(1) 将表中的数据按sex进行分组，并用统计函数进行统计
select sex,count(*),max(age),min(age),avg(age),sum(age) from student group by sex;
cout()：统计分组后，每组的总记录数；
(2)按sex进行分组之后并排序
select sex,count(*) from student group by sex asc;
(3)先按grade进行分组，再按sex进行分组
select *,count(*) from student group by grade,sex;
(4)group_connect(字段名) 可以对分组的结果的某个字段值进行字符串连接，保留该组某个字段的所有值
select grade,count(*),group_concat(name) from student group by grade;
+-------+----------+--------------------+
| grade | count(*) | group_concat(name) |
+-------+----------+--------------------+
| 3 | 4 | qiao,zhang,li,chen |
| 4 | 3 | li,zhao,qian |
+-------+----------+--------------------+
(5)with rollup 关键字，可以在每次分组之后，根据当前分组的字段进行统计，并向上一级进行汇报，可以和ifnull()混用
select grade,count(*) from student group by grade with rollup;
+-------+----------+
| grade | count(*) |
+-------+----------+
| 3 | 4 |
| 4 | 3 |
| NULL | 7 |
+-------+----------+
3 rows in set (0.00 sec)
select ifnull(grade,'总计'),count(*) from student group by grade with llup;
+------------------------+----------+
| ifnull(grade,'总计') | count(*) |
+------------------------+----------+
| 3 | 4 |
| 4 | 3 |
| 总计 | 7 |
+------------------------+----------+
3.having
可以把where看作where的子集，having有一些where之外的能力
(1)统计函数只能having使用
select grade,count(*) from student group by grade having count(*) >= 2;
(2)having可以使用字段别名,where不能
select grade,count(*) as total from student group by grade having total >= 2
4.order by + [asc/desc]
将表中的数据先按age升序排序(默认)，再按grade降序排序
select * from student order by age , grade desc;
5.limit
(1)用来限制长度，查询表中的指定条数据
select * from student limit 3;
(2)限制起始值，限制指定长度
select * from student limit 2,2;