第一范式：属性不可拆分
第二范式：非主键不能部分依赖组合主键
第三范式：不允许传递依赖
查询：
select * from student order by Snum desc;#按照序号降序排列
select * from student order by Sage asc;#按照年龄升序排列
select * from sc order by Snum,score desc ;#先按照序号升序排列（默认升序），再按照成绩降序排列
select * from student limit 3;#显示前3行
select * from student limit 2,3;#偏移2行再显示3行，即从第3行开始显示
select distinct Snum from sc ;#去重
聚合函数：
select count(*) from student;
max(),min(),avg(),#平均值
时间函数：
select datediff(now(),'1998-12-20'); #第一个参数减去第二个参数，返回出生了多少天
判断是否为空：第一个参数为null 则返回第二个；否则返回本身；0不是空
select ifnull(null,4) ;#结果为4
select ifnull(0,4);#结果为0
分组函数：其中条件语句用having，where只能用在表名后面
select snum,sum(score) from sc group by snum order by sum(score) desc; #每个人的总成绩
select snum,sum(score) from sc group by snum having sum(score)>200;#总成绩大于200
起别名：1.空格 2.as
运行顺序：from-> where->group by -> 显示（别名）->having ->order
select snum,sum(score) aa from sc where snum != 1 group by snum having aa > 200 order by sum(score) desc;
多表查询：
#笛卡尔积：
select * from student,sc where student.Snum = sc.snum;
关键字inner left right 要与表名相邻
#内联：
select * from student
inner join sc on student.Snum = sc.snum;
#外联
#左联：
select * from student
left join sc on student.Snum = sc.snum;
#右联：
select * from student
right join sc on student.Snum = sc.snum;
select嵌套：
select *,
(select score from sc where cnum = '01' and student.snum = sc.snum) sc01,
(select score from sc where cnum = '02' and student.snum = sc.snum) sc02
from student; #将sc表中的课程01，02加入表student
select * from
(
select *,
(select score from sc where cnum = '01' and student.snum = sc.snum) sc01,
(select score from sc where cnum = '02' and student.snum = sc.snum) sc02
from student
) a where sc01> sc02;#显示成绩sc01>sc02的人信息
select * from
(select *,
( select score from sc where cnum = '01' and sc.snum = student.snum) sc01,
ifnull(( select score from sc where cnum = '02' and sc.snum = student.snum),0) sc02
from student ) a
where sc01 > ifnull(sc02,-1)