面试必备杀技：SQL查询专项训练！

表结构及基本测试数据

#创建表及插入记录
 CREATE TABLE class (
   cid int(11) NOT NULL AUTO_INCREMENT,
   caption varchar(32) NOT NULL,
   PRIMARY KEY (cid)
 ) ENGINE=InnoDB CHARSET=utf8;INSERT INTO class VALUES
 (1, '三年二班'), 
 (2, '三年三班'), 
 (3, '一年二班'), 
 (4, '二年九班');CREATE TABLE course(
   cid int(11) NOT NULL AUTO_INCREMENT,
   cname varchar(32) NOT NULL,
   teacher_id int(11) NOT NULL,
   PRIMARY KEY (cid),
   KEY fk_course_teacher (teacher_id),
   CONSTRAINT fk_course_teacher FOREIGN KEY (teacher_id) REFERENCES teacher (tid)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO course VALUES
 (1, '生物', 1), 
 (2, '物理', 2), 
 (3, '体育', 3), 
 (4, '美术', 2);CREATE TABLE score (
   sid int(11) NOT NULL AUTO_INCREMENT,
   student_id int(11) NOT NULL,
   course_id int(11) NOT NULL,
   num int(11) NOT NULL,
   PRIMARY KEY (sid),
   KEY fk_score_student (student_id),
   KEY fk_score_course (course_id),
   CONSTRAINT fk_score_course FOREIGN KEY (course_id) REFERENCES course (cid),
   CONSTRAINT fk_score_student FOREIGN KEY (student_id) REFERENCES student(sid)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO score VALUES
 (1, 1, 1, 10),
 (2, 1, 2, 9),
 (5, 1, 4, 66),
 (6, 2, 1, 8),
 (8, 2, 3, 68),
 (9, 2, 4, 99),
 (10, 3, 1, 77),
 (11, 3, 2, 66),
 (12, 3, 3, 87),
 (13, 3, 4, 99),
 (14, 4, 1, 79),
 (15, 4, 2, 11),
 (16, 4, 3, 67),
 (17, 4, 4, 100),
 (18, 5, 1, 79),
 (19, 5, 2, 11),
 (20, 5, 3, 67),
 (21, 5, 4, 100),
 (22, 6, 1, 9),
 (23, 6, 2, 100),
 (24, 6, 3, 67),
 (25, 6, 4, 100),
 (26, 7, 1, 9),
 (27, 7, 2, 100),
 (28, 7, 3, 67),
 (29, 7, 4, 88),
 (30, 8, 1, 9),
 (31, 8, 2, 100),
 (32, 8, 3, 67),
 (33, 8, 4, 88),
 (34, 9, 1, 91),
 (35, 9, 2, 88),
 (36, 9, 3, 67),
 (37, 9, 4, 22),
 (38, 10, 1, 90),
 (39, 10, 2, 77),
 (40, 10, 3, 43),
 (41, 10, 4, 87),
 (42, 11, 1, 90),
 (43, 11, 2, 77),
 (44, 11, 3, 43),
 (45, 11, 4, 87),
 (46, 12, 1, 90),
 (47, 12, 2, 77),
 (48, 12, 3, 43),
 (49, 12, 4, 87),
 (52, 13, 3, 87); CREATE TABLE student(
   sid int(11) NOT NULL AUTO_INCREMENT,
   gender char(1) NOT NULL,
   class_id int(11) NOT NULL,
   sname varchar(32) NOT NULL,
   PRIMARY KEY (sid),
   KEY fk_class (class_id),
   CONSTRAINT fk_class FOREIGN KEY (class_id) REFERENCES class (cid)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO student VALUES
 (1, '男', 1, '理解'), 
 (2, '女', 1, '钢蛋'), 
 (3, '男', 1, '张三'), 
 (4, '男', 1, '张一'), 
 (5, '女', 1, '张二'), 
 (6, '男', 1, '张四'), 
 (7, '女', 2, '铁锤'), 
 (8, '男', 2, '李三'), 
 (9, '男', 2, '李一'), 
 (10, '女', 2, '李二'), 
 (11, '男', 2, '李四'), 
 (12, '女', 3, '如花'), 
 (13, '男', 3, '刘三'), 
 (14, '男', 3, '刘一'), 
 (15, '女', 3, '刘二'), 
 (16, '男', 3, '刘四');CREATE TABLE teacher(
   tid int(11) NOT NULL AUTO_INCREMENT,
   tname varchar(32) NOT NULL,
   PRIMARY KEY (tid)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO teacher VALUES
 (1, '张磊老师'), 
 (2, '李平老师'), 
 (3, '刘海燕老师'), 
 (4, '朱云海老师'), 
 (5, '李杰老师');

需要完成的查询问题：

1、查询所有的课程的名称以及对应的任课老师姓名

2、查询学生表中男女生各有多少人

3、查询物理成绩等于100的学生的姓名

4、查询平均成绩大于八十分的同学的姓名和平均成绩

5、查询所有学生的学号，姓名，选课数，总成绩

6、 查询姓李老师的个数

7、 查询没有报李平老师课的学生姓名

8、 查询物理课程比生物课程高的学生的学号

9、 查询没有同时选修物理课程和体育课程的学生姓名

10、查询挂科超过两门(包括两门)的学生姓名和班级
、查询选修了所有课程的学生姓名

12、查询李平老师教的课程的所有成绩记录
 
13、查询全部学生都选修了的课程号和课程名

14、查询每门课程被选修的次数

15、查询之选修了一门课程的学生姓名和学号

16、查询所有学生考出的成绩并按从高到低排序（成绩去重）

17、查询平均成绩大于85的学生姓名和平均成绩

18、查询生物成绩不及格的学生姓名和对应生物分数

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

参考解决答案：

#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 性别,
     count(1) 人数
 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
     student.sname,
     t1.avg_num
 FROM
     student
 INNER JOIN (
     SELECT
         student_id,
         avg(num) AS avg_num
     FROM
         score
     GROUP BY
         student_id
     HAVING
         avg(num) > 80
 ) AS t1 ON student.sid = t1.student_id;

#5、查询所有学生的学号，姓名，选课数，总成绩(注意：对于那些没有选修任何课程的学生也算在内)

SELECT
     student.sid,
     student.sname,
     t1.course_num,
     t1.total_num
 FROM
     student
 LEFT JOIN (
     SELECT
         student_id,
         COUNT(course_id) course_num,
         sum(num) total_num
     FROM
         score
     GROUP BY
         student_id
 ) AS t1 ON student.sid = t1.student_id;

#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(找出报名李平老师课程的学生，然后取反就可以)

SELECT
     student.sname
 FROM
     student
 WHERE
     sid NOT IN (
         SELECT DISTINCT
             student_id
         FROM
             score
         WHERE
             course_id IN (
                 SELECT
                     course.cid
                 FROM
                     course
                 INNER JOIN teacher ON course.teacher_id = teacher.tid
                 WHERE
                     teacher.tname = '李平老师'
             )
     );

#8、 查询物理课程比生物课程高的学生的学号(分别得到物理成绩表与生物成绩表，然后连表即可)

SELECT
     t1.student_id
 FROM
     (
         SELECT
             student_id,
             num
         FROM
             score
         WHERE
             course_id = (
                 SELECT
                     cid
                 FROM
                     course
                 WHERE
                     cname = '物理'
             )
     ) AS t1
 INNER JOIN (
     SELECT
         student_id,
         num
     FROM
         score
     WHERE
         course_id = (
             SELECT
                 cid
             FROM
                 course
             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(没有同时选修指的是选修了一门的，思路是得到物理+体育课程的学生信息表，然后基于学生分组，统计count(课程)=1)

SELECT
     student.sname
 FROM
     student
 WHERE
     sid IN (
         SELECT
             student_id
         FROM
             score
         WHERE
             course_id IN (
                 SELECT
                     cid
                 FROM
                     course
                 WHERE
                     cname = '物理'
                 OR cname = '体育'
             )
         GROUP BY
             student_id
         HAVING
             COUNT(course_id) = 1
     );

#10、查询挂科超过两门(包括两门)的学生姓名和班级(求出<60的表，然后对学生进行分组，统计课程数目>=2)

SELECT
     student.sname,
     class.caption
 FROM
     student
 INNER JOIN (
     SELECT
         student_id
     FROM
         score
     WHERE
         num < 60
     GROUP BY
         student_id
     HAVING
         count(course_id) >= 2
 ) AS t1
 INNER JOIN class ON student.sid = t1.student_id
 AND student.class_id = class.cid;

#11、查询选修了所有课程的学生姓名(先从course表统计课程的总数，然后基于score表按照student_id分组，统计课程数据等于课程总数即可)

SELECT
     student.sname
 FROM
     student
 WHERE
     sid IN (
         SELECT
             student_id
         FROM
             score
         GROUP BY
             student_id
         HAVING
             COUNT(course_id) = (SELECT count(cid) FROM course)
     );

#12、查询李平老师教的课程的所有成绩记录

SELECT
     *
 FROM
     score
 WHERE
     course_id IN (
         SELECT
             cid
         FROM
             course
         INNER JOIN teacher ON course.teacher_id = teacher.tid
         WHERE
             teacher.tname = '李平老师'
     );

#13、查询全部学生都选修了的课程号和课程名(取所有学生数，然后基于score表的课程分组，找出count(student_id)等于学生数即可)

SELECT
     cid,
     cname
 FROM
     course
 WHERE
     cid IN (
         SELECT
             course_id
         FROM
             score
         GROUP BY
             course_id
         HAVING
             COUNT(student_id) = (
                 SELECT
                     COUNT(sid)
                 FROM
                     student
             )
     );

#14、查询每门课程被选修的次数

SELECT
     course_id,
     COUNT(student_id)
 FROM
     score
 GROUP BY
     course_id;

#15、查询之选修了一门课程的学生姓名和学号

SELECT
     sid,
     sname
 FROM
     student
 WHERE
     sid IN (
         SELECT
             student_id
         FROM
             score
         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,
     t1.avg_num
 FROM
     student
 INNER JOIN (
     SELECT
         student_id,
         avg(num) avg_num
     FROM
         score
     GROUP BY
         student_id
     HAVING
         AVG(num) > 85
 ) t1 ON student.sid = t1.student_id;

#18、查询生物成绩不及格的学生姓名和对应生物分数

SELECT
     sname 姓名,
     num 生物成绩
 FROM
     score
 LEFT JOIN course ON score.course_id = course.cid
 LEFT JOIN student ON score.student_id = student.sid
 WHERE
     course.cname = '生物'
 AND score.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
                     course.cid
                 FROM
                     course
                 INNER JOIN teacher ON course.teacher_id = teacher.tid
                 WHERE
                     teacher.tname = '李平老师'
             )
         GROUP BY
             student_id
         ORDER BY
             AVG(num) DESC
         LIMIT 1
     );