SQL多表查询，视图、函数、存储过程

1. 索引：提高查询速度
   创建、删除索引

   creat index index_sname on student(sname);
   drop index index_sname on student;

2. SQL支持关系数据库三级模式结构
   

3. 等值链接，把不同表通过相同的属性连接到一起
   把user表和menu表连接到一起

   select user.id,user.name,menu.mname from user,menu
   where user.id=menu.id

4. 内连接：inner join 返回两个表的交集部分
   将student表和sc表通过学号连接起来

   select s.sno,s.sname,sc.grade from student as s inner join
   sc on s.sno=sc.sno

5. 左连接：left join 左表的全部都会表示出来
   将student表和sc表左连接

   select s.sno,s.sname,sc.cno,sc.grade from student as s 
   left join sc s.sno=sc.sno

   所有选课的学生的成绩信息

   select s.sno,s.sname,sc.cno,sc.grade from student as s
   left jion sc s.sno=sc.sno where sc.cno is not null

   查出每个学生的总分

   select s.sno,s.sname,sc.cno,sum(sc.grade) from student as s
   left jion sc s.sno=sc.sno where sc.cno is not null
   group by s.sno

6. 视图是一张假表，可以修改，不能增加删除
   创建一个视图，显示学号，姓名，课程名，成绩信息

   creat view student_info as
   SELECT s.sno,s.sname,c.cname,sc.grade from sc INNER JOIN student as s
   INNER JOIN course as c on s.sno=sc.sno and c.cno=sc.cno

   查询李丽的数学成绩

   select * from student_info as so where so.sname='李丽'
   and so.cname='数学'

7. 函数
   创建一个函数返回学生地址

   delimiter &&
   creat function getAddress(myname varchar(12))
   returns varchar(12)
   begin
       return(select sprovince from student where sname=myname);
   end &&
   select getAddress('李丽');

8. 存储过程是为了完成特定功能的SQL语句集
   创建一个存储过程可以通过名字获取地址

   delimiter &&
   creat procedure getAddressByName(in myname VARCHAR(12),out addr VARCHAR(12))
   begin
       select sprovince into addr from student where sname=myname;
   end &&
   @name='李丽';
   @addr='';
   call getAddressByName(@name,@addr);
   select @addr;

   输入姓名和学科查出成绩，并判断等级

   delimiter &&
   CREATE procedure getScoreLevel(in myname VARCHAR(12),inout cname_res VARCHAR(12))
   BEGIN
   DECLARE score int default(0);
   DECLARE sid VARCHAR(12) default('');
   DECLARE cid VARCHAR(12) default('');
   
   select sno into sid from student where sname=myname;
   SELECT cno into cid from course where cname=cname_res;
   SELECT grade into score from sc where sc.sno=sid and sc.cno=cid;
   if score<60 THEN
           set cname_res='不及格';
   elseif score>=60 and score<80 THEN
           set cname_res='及格';
   else 
           set cname_res='优秀';
   end IF;
   END &&
   set @name='李丽';
   set @cname='数学';
   call getScoreLevel(@name,@cname);
   SELECT @cname;

   循环加分直到所有人及格

   delimiter &&
   CREATE PROCEDURE setAllPass(inout num int)
   BEGIN
   declare min_score int default(-1);
   SELECT min(grade) into min_score from sc;
   if min_score>=60 then
           set num=1;
   ELSE
           while min_score<60 do
                   UPDATE sc set grade=grade+num where grade=min_score;
                   SELECT min(grade) into min_score from sc;
           end WHILE;
   end if;
   END &&
   set @num=4;
   call setAllPass(@num);