常用SQL语句查询分享

--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'
)
)
);