char(n)固定长度

Select 

索引

select * from student

where exists

(select id from scores s where s.id=s.id);

selext *from student st

(select id from score s where s.id=st.id

左链

1.....内连接

2...每个学生选修科目名称和成绩

select* from select from 

where s.score

select id,score,case when score<60

select sex, if (sex='男',0,1) from student;

select sum(if(sex='男'm1,0)),sum(if(sex='‘ from student;

select id,ifnull(scroe,0) from scores;

order by avg(score) desclimit 1;

select sname ,sex from student

where address='wudang' or address='huashan'

whrere  address in ('wudang','huashan')

select * from student

where sname like 'zhang%' or address like'_li%;

select   from scores where id。3；j'k

select  id，namefrom student；

select distinact id ,name from student;

select id as njNNnjnknjnNnnnnjnj

DESCRIBE STUDENT;

DESC STUDENT;

 show  

ADD 列名    列类型