mysql 连左查询

create table teacher(
     tid int unsigned auto_increment primary key,
     tname varchar(30) not null
)engine=myisam charset =utf8;
insert into teacher values(null,'王老师');
insert into teacher values(null,'李老师');
insert into teacher values(null,'方老师');
select * from teacher;
-- 按照降序排序
select * from teacher order by tid desc;
-- 按照升序排列
select * from teacher order by convert (tname using gbk) asc;
create table student(
sid int unsigned auto_increment primary key,
sname varchar(10),
sgender enum('男','女','保密'),
sscore tinyint unsigned,
sdept varchar(50),
stid int unsigned  --  必须和老师的tid要用一致的类型
)engine=myisam auto_increment=201501 charset utf8;
insert into student values (null,'李四','男',90,'法学',1);
insert into student values (null,'李二','男',null,'法学',1);
insert into student values (null,'王二','男',50,'法学',3);
select * from student;
-- 统计总共有多少人数
select count(*) 人数 from student;
-- 统计总共有多少男生人数
 select count(*) 男生人数 from student where sgender ='男';
 -- 统计分数为空的学生有多少人
 select count(*) from student where sscore is null;
 -- 所有学生的平均,最高分，最低分， 成绩
 select count(*) 总人数 ,avg(sscore) 平均分 ,max(sscore),min(sscore)from student;
 -- 统计各专业的人数
select sdept 专业,count(*) 人数,max(sscore) 最高分,min(sscore) 最低分
from student where sgender='男' group by sdept order by avg(sscore) desc;
-- 清除重复的专业从student表里面
select distinct sdept from student;
-- 查找最高分和最低分
select * from student where sscore in(select max(sscore)from student)
union
select * from student
select * from teacher;
where sscore in(select min(sscore)  from student)
--  -- 显示学生学号 姓名 成绩 代课老师姓名
select sid,sname,sscore,tname  代课老师  from student,teacher where stid = tid;
-- 连左查询  和上面同样的效果
select sid,sname,sscore,tname from student left join teacher on stid=tid;
-- 显示老师id 姓名 学生 成绩
select tid,tname ,sname,sscore from teacher lef join student on stid =tid
 order by tname asc;