MySql数据库基本select查询语句练习题，初学者易懂。

在数据库建立四个表：分别为

student（sid,sname,sage,ssex）

teacher(tid,tname)

course(cid,cname,tid)

sc(sid,cid,score)

-- 1、查询“001”课程比"002"课程成绩高的所有学生的学号。

select a.sid FROM
(select * from sc where cid="001") as a,
(select * from sc where cid = "002")as b
where a.sid = b.sid and a.score>b.score

-- 2、查询平均成绩大于60分的同学的学号和平均成绩

select sid,avg(score) 
FROM sc
GROUP BY sid
HAVING avg(score)>=70;

-- 3、查询所有的同学的学号、姓名、选课数、总成绩

select student.sid, sname,COUNT(*),sum(score)
from sc,student
where sc.sid = student.sid 
GROUP BY sid;

-- 4、查询姓“李”的老师的个数

select COUNT(*)
from teacher
where tname LIKE '大%';

-- 5、查询没学过“叶平“老师的课程的同学的学号、姓名

SELECT sid,sname
from student
where sid NOT in(select sid
from sc,course,teacher
where sc.cid = course.cid and teacher.tid = course.tid and teacher.tname = "叶良辰");

--  6、查询所有    课程有挂科的同学的学号、姓名

SELECT sid ,sname
from student
where 
sid in (select sid from sc) AND
sid not in(select sid from sc where score<60 GROUP BY sid);

--  7、查询至少一门课与学号为“2”的同学所学相同的学生的学号和姓名

SELECT DISTINCT(sc.sid)
from sc,student
where student.sid = sc.sid and cid in(select cid from sc where sc.sid='2') and student.sid<>'2';

-- 8、统计列印各科成绩，各分数段人数：课程ID、课程名称，100-85,85-70,70-60,<60

select sc.cid as '课程ID',cname as '课程名称',
SUM(case WHEN score between 85 and 100 then 1 else 0 end) as '85-100',
SUM(case WHEN score between 70 and 84 then 1 else 0 end) as '70-84',
SUM(case WHEN score between 69 and 60 then 1 else 0 end) as '60-69',
SUM(case WHEN score between 0 and 100 then 59 else 0 end) as '0-59'
from course,sc
where sc.cid=course.cid
GROUP BY sc.cid;

--  9、查询每门课程的课程名和选修的学生数

select cname,count(*)
from sc,course
where course.cid=sc.cid 
group by sc.cid;

-- 10、查询出只选修了一门课程的全部同学的学号、姓名

select sc.sid,sname
from sc,student
where student.sid=sc.sid
GROUP BY (sc.sid)
HAVING COUNT(*)=1;

-- 11、查询男生、女生的人数

(select "男生" AS "性别",count(*) from student where ssex="男")
UNION
(select "女生" as "性别", count(*) from student where ssex="女");

--  12、查询姓“李”的师生名单

(select sname as '名单' from student where sname like '李%')
union
(select tname as '名单' from teacher where tname like "李%");

在数据库建立三个表：

学生表：student（sno,sname,sage,ssex,sdept）==（学号，姓名，年龄，性别，系别）

课程表：course(cno,cname,credit)==(课程号，课程名，学分)

选课表：sc(sno,cno,grade)===（学号，课程号，成绩）

-- 写出选修了数据结构的同学的学号和姓名

select *
from student
where sno in(select sno
 from sc,course 
where sc.cno=course.cno and cname='数据结构' 
);

-- 1.统计每门课的选课人数，包括没有人选的课程，列出课程号及选课情况，其中选课情况为，如果此门课的选课人数超过100
-- 人，则显示人多，40-100一般 1-40人好，无人选

select  course.cno ,
case 
when (count(*)>=40 and count(*)<=100) then  '较多' 
when (count(*)>1 and count(*)<40) then '较少'
else '无人选' end as '选课情况'
from course left join sc
on course.cno=sc.cno
GROUP BY cno;

--  2.查询计算机有哪些学生没有选课，列出姓名和学号（用外连接）
select sname
from student left join  sc on
sc.sno=student.sno
where sdept="计科" and sc.sno is null;

2>-- 成绩小于60的学生姓名,课程,成绩

select sname,cname,grade
from student,sc,course
where student.sno=sc.sno and sc.cno=course.cno
and grade<60;

-- 3. 统计每个学生的选课人数和考试总成绩，并按照选课门数升序排列
select sno,count(*)'选课门数' ,sum(grade)'总成绩'
from sc
GROUP BY sno
ORDER BY count(*) DESC;