一、查询实例
1、select 查询字段 from 表名 where 查询条件
举例说明：
表名：student
表中字段：id,name,age,score,subject,class
表中数据：
(1,"Sheldon",18,99,"Maths", "one"),
(1,"Sheldon",18,90,"Chinese","one"),
(1,"Sheldon",18,85,"English","one"),
(2,"Kim",20,89,"Maths","two"),
(2,"Kim",20,95,"Chinese","two"),
(2,"Kim",20,92,"English","two");
1）查询出某条件下的所有信息
# 查询出表student中age为18的所有学生信息
select * from student where age=18
2）求和
# 计算并查询出表student中name为Sheldon的学生的总分数
select sum(score) from student where name="Sheldon"
3）求平均数
# 计算并查询出表student中name为Sheldon的学生的平均分数
select avg(score) from student where name="Sheldon"
4）求最小值：
# 计算并查询出表student中name为Sheldon的学生的最低分数
select min(score) from student where name="Sheldon"
5）求最大值
# 计算并查询出表student中name为Sheldon的学生的最高分数
select max(score) from student where name="Sheldon"
6）对查询结果排列
#查询出表student中name为Sheldon的学生的信息，且按照分数倒序排列（desc是逆序排序，asc是正序排序，asc可省略不写）
select * from student where name="Sheldon" order by score desc
7）求总数（未去重）
# 计算并查询出class为one的班级学生总数（实际上只有一个学生）
select count(*) from student from where class="one"
8）求总数（去重）
# 去重统计出class为one的班级学生总数
select count(distinct id) from student
9）根据一个或多个列表字段进行分组统计查询：
# 统计每个班19岁以上的学生人数（执行顺序where过滤->分组->聚合函数）
select class,count(distinct age) from student where age>19 group by class
表student如下：
10）多表连接查询：
表student如下：
表info如下：
# 两个表连接，并去掉重复的列
select a.*,b.address from student a join info b on a.id=b.id
11）嵌套查询：
# 查询出所有学生的id，name和address
select a.id,a.name,(select b.address from info b where a.id=b.id) address from student a
或者也可以用：
select a.id,a.name,b.address from student a,info b where a.id=b.id
二、插入、修改和删除实例
1.插入：
# 插入多条数据
insert into student values
(1,"Sheldon",18,99,"Maths", "one"),
(1,"Sheldon",18,90,"Chinese","one"),
(1,"Sheldon",18,85,"English","one"),
(2,"Kim",20,89,"Maths","two"),
(2,"Kim",20,95,"Chinese","two"),
(2,"Kim",20,92,"English","two");
2.修改：
# 修改Kim的Maths分数为92
update student set score=92 where name="Kim" and subject="Maths"
3.删除：
# 删除Kim的English信息
delete from student where name="Kim" and subject="English"
