--创建数据库(文件：主要数据文件mdf==1，次要数据文件ndf>=0，日志文件ldf>=1)
--文件组：当1mdf,5个ndf(1,2,2),10个ldf(3,3,4),将它们分成多个组存放
CREATE database studb;
--创建表teacher,student
create table teacher ( tid int(10) primary key auto_increment, tname varchar(20), tage int(10) ); use studb; create table student ( sid int(10) primary key auto_increment, sname varchar(20), sage int(10), tid int(10) REFERENCES teacher(tid) );
--外键约束：你问张三的老师是谁？？
--select teacher.tname from teacher,student where student.sname = '张三' select t.tname from teacher t,student s where s.sname = '张三' and t.tid = s.tid
--创建课程表
create table course ( cid int(10) primary key, cname varchar(20), tid int(10) REFERENCES teacher(tid) );
--创建分数表
create table sc ( scid int(10) primary key, sid int(10) REFERENCES student(sid), cid int(10) REFERENCES course(cid), score int(10) );
--联合查询：等值查询
--1..
select c.cname from course c,student s,sc where s.sname = '小张' and s.sid = sc.sid and c.cid = sc.cid;
--2..
select sname from student s,course c,sc where c.cname='android' and sc.score>=60 and s.sid = sc.sid and c.cid = sc.cid;
--3..
--子查询：当条件也要查询的时候,我只知道学号，我不知道"小张"这个字段,那你知道小张的学号 吗
delete from sc where sid = (select sid from student where sname = '小张');
--子查询中间的符号一定是父查询与子查询两张表关联的字段(一般是主外键)
--4..
update sc set score=score+5 where cid=????; select tid from teacher where tname='李老师' ==1 select cname from course where tid = 1 ==课程名字，李老师教的 select cid from course where cname='android' ==课程ID update sc set score=score+5 where cid= ( select cid from course where cname= ( select cname from course where tid = ( select tid from teacher where tname='李老师' ) ) );