数据库之多表查询

1、内连接：把两张表有对应关系的记录连接成一张虚拟表
select * from emp inner join dep on emp.dep_id = dep.id;

#应用：
    select * from emp,dep where emp.dep_id = dep.id and dep.name = "技术"; # 不要用where做连表的活

    select * from emp inner join dep on emp.dep_id = dep.id
        where dep.name = "技术"
    ;

2、左连接：在内连接的基础上，保留左边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id;


3、右连接：在内连接的基础上，保留右边没有对应关系的记录
select * from emp right join dep on emp.dep_id = dep.id;


4、全连接：在内连接的基础上，保留左、右边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;



#补充：多表连接可以不断地与虚拟表连接

查找各部门最高工资
select t1.* from emp as t1
inner join
(select post,max(salary) as ms from emp group by post) as t2
on t1.post = t2.post
where t1.salary = t2.ms

子查询：把一个查询语句用括号括起来，当做另一条查询语句的条件去用，称为子查询
 select emp.name from emp inner join dep on emp.dep_id=dep.id where dep.name=‘技术‘

 select name from emp where dep_id=
(select id from dep where name=‘技术‘)；

查询平均年龄在25以上的部门
 select name from dep where id in
（select dep_id from emp group by dep_id having avg（age）>25）
 select dep.name from emp inner join dep on emp.dep_id=dep.id
   group by dep.name
   having avg(age)>25;
查看不足2个人的部门名（子查询得到的是有人的部门id）
 select * from emp where exists（select id from dep where id>3）;
查询每一个部门最新入职的那个员工
 select t1.id,t1.name,t1.post,t1.hire_date,t2.post,t2.max_date from emp as t1 inner join (select post,max(hire_date) as max_date from emp group by post) as t2
 on t1.post=t2.post
 where t1.hire_date=t2.max_date
;

1 创建班级表：cid,caption
    学生表：sid，sname，gender，class_id
2 插入班级：
        1    linux一班
        2    linux二班
        3    linux三班
        4    linux四班
        5    python一班
        6    python二班
        7    python三班
        8    python四班
3 插入学生（都在linux2班）：
            (‘王五‘, ‘女‘),
            (‘王6蛋‘, ‘男‘),
            (‘王7蛋‘, ‘女‘),
            (‘王8‘, ‘女‘) ;
4 插入学生：（都在linux3班）：
                (‘刘五‘, ‘女‘),
                (‘刘6‘, ‘男‘),
                (‘刘7‘, ‘女‘),
                (‘刘8‘, ‘女‘) ;
5 插入学生：（都在python1班班）：
                   (‘李1‘, ‘女‘,
                (‘李2‘, ‘男‘),
                (‘李3‘, ‘女‘),
                (‘李4‘, ‘女‘) ;
6 插入学生：（都在python2班班）：
                (‘岳1‘, ‘女‘),
                (‘岳2岳‘, ‘男‘),
                (‘岳3岳‘, ‘女‘),
                (‘岳4岳‘, ‘女‘) ;
7 查询linux1班学生个数
8 查询linux2班学生个数
9  查询linux3班学生个数
10 查询linux1班 的班级id和所有人的名字，把名字列表的列名命名为names
11 查询python1班 班级id，学生id最大的人id，所有人的名字，把名字列表的列名命名为names
12 查询名字里有1的人的姓名，性别
13 查询名字叫王（某）某不定：如王4，王2，不包括王2蛋，人的姓名，性别
14 查询姓王的人的id，性别，姓名，班级id，只取第一条
15 查询所有学生，按学生id倒排序
16 查询所有学生，按学生姓名正排序
17 查询所有学生，按班级倒排序，sid正排序
18 查询姓王，名字只有俩字的人名和sid，按sid正序排列
19 查询姓王的人名和sid，按sid正序排列
20 查询名字以蛋结尾的人名和id，只取id最大的一条



INSERT INTO student (sname, gender, class_id)
VALUE
    (‘王五‘, ‘女‘, 2),
    (‘王6蛋‘, ‘男‘, 2),
    (‘王7蛋‘, ‘女‘, 2),
    (‘王8‘, ‘女‘, 2) ;


INSERT INTO student (sname, gender, class_id)
VALUE
    (‘刘五‘, ‘女‘, 3),
    (‘刘6‘, ‘男‘, 3),
    (‘刘7‘, ‘女‘, 3),
    (‘刘8‘, ‘女‘, 3) ;


INSERT INTO student (sname, gender, class_id)
VALUE
    (‘李1‘, ‘女‘, 5),
    (‘李2‘, ‘男‘, 5),
    (‘李3‘, ‘女‘, 5),
    (‘李4‘, ‘女‘, 5) ;


INSERT INTO student (sname, gender, class_id)
VALUE
    (‘岳1‘, ‘女‘, 6),
    (‘岳2岳‘, ‘男‘, 6),
    (‘岳3岳‘, ‘女‘, 6),
    (‘岳4岳‘, ‘女‘, 6) ;

-- 查询linux1班学生个数
SELECT count(*) from student GROUP BY class_id HAVING class_id=1;
SELECT count(*) from student where class_id=1


select count(*) from student where gender=‘女‘;

--  查询linux1班 的班级id和所有人的名字，把名字列表的列名命名为names
SELECT class_id,GROUP_CONCAT(sname) as names from student GROUP BY class_id HAVING class_id=1;
-- 查询python1班 班级id，学生id最大的人id，所有人的名字，把名字列表的列名命名为names

SELECT class_id,max(sid),GROUP_CONCAT(sname) as names from student GROUP BY class_id HAVING class_id=5;

-- 查询名字里有1的人的姓名，性别
SELECT sname,gender from student where sname like ‘%1%‘

-- 查询名字叫王（某）某不定：如王4，王2，不包括王2蛋，人的姓名，性别
SELECT sname,gender from student where sname like ‘王_‘
-- 查询姓王的人的id，性别，姓名，班级id，只取第一条
select * from student where sname like ‘王%‘ LIMIT 1;
-- 查询所有学生，按学生id倒排序
SELECT* from student ORDER BY sid desc;
-- 查询所有学生，按学生姓名正排序
SELECT* from student ORDER BY sname asc;
-- 查询所有学生，按班级倒排序，sid正排序
SELECT  * from student ORDER BY class_id desc,sid asc;

-- 查询姓王，名字只有俩字的人名和sid，按sid正序排列
SELECT sname,sid from student where sname like ‘王_‘ ORDER BY sid 
-- 查询姓王的人名和sid，按sid正序排列
SELECT sname,sid from student where sname like ‘王%‘ ORDER BY sid 

-- 查询名字以蛋结尾的人名和id，只取id最大的一条
SELECT sid,sname from student where sname like ‘%蛋‘ ORDER BY sid desc limit 1;

1、查询所有的课程的名称以及对应的任课老师姓名
select course.cname,teacher.tname from course INNER JOIN teacher on course.teacher_id=teacher.tid;
2、查询学生表中男女生各有多少人
select gender,count(sid) from student group by gender;
3、查询物理成绩等于100的学生的姓名
 
select student.sname from student WHERE sid IN
(select student_id from score INNER JOIN course on score.course_id=course.cid where course.cname=‘物理‘ and score.num=100);
4、查询平均成绩大于八十分的同学的姓名和平均成绩
select sname,avg(num) from student inner join score on student.sid = score.student_id 
group by student_id having avg(num)>80;

5、查询所有学生的学号，姓名，选课数，总成绩
select student.sid,sname,count(course_id),SUM(num) from student INNER JOIN score on student.sid=score.student_id GROUP BY student.sid;


6、 查询姓李老师的个数
select count(tid) from teacher where tname like ‘李%‘;
7、 查询没有报李平老师课的学生姓名
select sname from student where sid not in
(select student_id from score where course_id in(
select cid from course where teacher_id=(select tid from teacher where tname=‘李平老师‘)));

8、 查询物理课程比生物课程高的学生的学号
select t1.student_id  学号 from 
(select student_id ,num from score inner join course on score.course_id=course.cid  where cname=‘物理‘ )as t1
inner join 
(select student_id , num from score inner join course on score.course_id=course.cid  where cname = ‘生物‘) as t2
on t1.student_id = t2.student_id
where t1.num>t2.num;


9、 查询没有同时选修物理课程和体育课程的学生姓名
select sname from student inner join score on student.sid = score.student_id 46 join course on course.cid=score.course_id and cname in (‘物理‘,‘体育‘) 47 group by student_id having count(course_id)!=2;

10、查询挂科超过两门(包括两门)的学生姓名和班级
select sname 姓名,caption 班级 from student inner join score on student.sid = score.course_id
join class on class.cid = score.course_id
 where num<60 group by student_id having count(course_id)>=2;

11 、查询选修了所有课程的学生姓名
(select student_id,count(course_id) from score group by student_id having count(course_id) = (
select count(cid) from course)) as t1
on t1.student_id = student.sid;

12、查询李平老师教的课程的所有成绩记录
 
select num from score inner join course on course.cid=score.course_id
join teacher on teacher.tid=course.teacher_id
where tname = ‘李平老师‘;
13、查询全部学生都选修了的课程号和课程名
select cid,cname from course student;
14、查询每门课程被选修的次数
select course.cname,count(student_id) from score INNER JOIN course on score.course_id=course.cid
GROUP BY course_id;

15、查询之选修了一门课程的学生姓名和学号
select sname 姓名,student_id 学号 from student inner join score on student.sid = score.student_id
group by student_id having count(course_id)=1;

16、查询所有学生考出的成绩并按从高到低排序（成绩去重）
select DISTINCT num from score ORDER BY num desc;
17、查询平均成绩大于85的学生姓名和平均成绩
select sname 姓名,avg(num) 平均成绩 from student inner join score on student.sid = score.student_id
group by student_id having avg(num)>85;

18、查询生物成绩不及格的学生姓名和对应生物分数
select student.sname ,num 生物成绩 from student inner join score on student.sid = score.student_id
join course on course.cid=score.course_id
where cname=‘生物‘ and num<60;

19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名
select sname from student where sid=(
select student_id from score where course_id in (
select cid from course where teacher_id=(select tid from teacher where tname=‘李平老师‘)
) group by student_id order by avg(num) desc limit 1
)

20、查询每门课程成绩最好的前两名学生姓名

1、查询所有的课程的名称以及对应的任课老师姓名
select course.cname,teacher.tname from course INNER JOIN teacher on course.teacher_id=teacher.tid;
2、查询学生表中男女生各有多少人
select gender,count(sid) from student group by gender;
3、查询物理成绩等于100的学生的姓名
 
select student.sname from student WHERE sid IN
(select student_id from score INNER JOIN course on score.course_id=course.cid where course.cname=‘物理‘ and score.num=100);
4、查询平均成绩大于八十分的同学的姓名和平均成绩
select sname,avg(num) from student inner join score on student.sid = score.student_id 
group by student_id having avg(num)>80;

5、查询所有学生的学号，姓名，选课数，总成绩
select student.sid,sname,count(course_id),SUM(num) from student INNER JOIN score on student.sid=score.student_id GROUP BY student.sid;


6、 查询姓李老师的个数
select count(tid) from teacher where tname like ‘李%‘;
7、 查询没有报李平老师课的学生姓名
select sname from student where sid not in
(select student_id from score where course_id in(
select cid from course where teacher_id=(select tid from teacher where tname=‘李平老师‘)));

8、 查询物理课程比生物课程高的学生的学号
select t1.student_id  学号 from 
(select student_id ,num from score inner join course on score.course_id=course.cid  where cname=‘物理‘ )as t1
inner join 
(select student_id , num from score inner join course on score.course_id=course.cid  where cname = ‘生物‘) as t2
on t1.student_id = t2.student_id
where t1.num>t2.num;


9、 查询没有同时选修物理课程和体育课程的学生姓名
select sname from student inner join score on student.sid = score.student_id 46 join course on course.cid=score.course_id and cname in (‘物理‘,‘体育‘) 47 group by student_id having count(course_id)!=2;

10、查询挂科超过两门(包括两门)的学生姓名和班级
select sname 姓名,caption 班级 from student inner join score on student.sid = score.course_id
join class on class.cid = score.course_id
 where num<60 group by student_id having count(course_id)>=2;

11 、查询选修了所有课程的学生姓名
(select student_id,count(course_id) from score group by student_id having count(course_id) = (
select count(cid) from course)) as t1
on t1.student_id = student.sid;

12、查询李平老师教的课程的所有成绩记录
 
select num from score inner join course on course.cid=score.course_id
join teacher on teacher.tid=course.teacher_id
where tname = ‘李平老师‘;
13、查询全部学生都选修了的课程号和课程名
select cid,cname from course student;
14、查询每门课程被选修的次数
select course.cname,count(student_id) from score INNER JOIN course on score.course_id=course.cid
GROUP BY course_id;

15、查询之选修了一门课程的学生姓名和学号
select sname 姓名,student_id 学号 from student inner join score on student.sid = score.student_id
group by student_id having count(course_id)=1;

16、查询所有学生考出的成绩并按从高到低排序（成绩去重）
select DISTINCT num from score ORDER BY num desc;
17、查询平均成绩大于85的学生姓名和平均成绩
select sname 姓名,avg(num) 平均成绩 from student inner join score on student.sid = score.student_id
group by student_id having avg(num)>85;

18、查询生物成绩不及格的学生姓名和对应生物分数
select student.sname ,num 生物成绩 from student inner join score on student.sid = score.student_id
join course on course.cid=score.course_id
where cname=‘生物‘ and num<60;

19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名
select sname from student where sid=(
select student_id from score where course_id in (
select cid from course where teacher_id=(select tid from teacher where tname=‘李平老师‘)
) group by student_id order by avg(num) desc limit 1
)

20、查询每门课程成绩最好的前两名学生姓名