MySQL查询（下）

存在以下四个数据库表
学生表：student1(sid,sname) Sid学号,sname学生姓名
教师表：teacher(tid,tname) tid教师编号，tname教师名字
课程表：course(cid,cname,tid) cid课程编号,cname课程姓名,tid教师编号
成绩表：cj(sid,cid,score) sid学号，cid选修课程编号，score 成绩

--1、查询所有同学的学号、姓名、选课数、总成绩

查询：select 字段1,字段2[*] from 表名

SELECT student1.sid,student1.sname,count(cj.cid),SUM(cj.score) 
FROM student1,cj 
WHERE student1.sid=cj.sid 
GROUP BY cj.sid,student1.sname

--2、查询平均成绩大于60分的同学的学号和平均成绩

max() 求最大
min() 求最小值
avg() 求平均值
sum() 求和
count()求个数

select sid,SUM(cj.score)/COUNT(cj.cid) from cj 
group by sid
having  max(score)>60;

--3、查询选修的课程不少于3门的每个学生的编号，学生姓名

select student1.sid,student1.sname,COUNT(cj.cid) from student1,cj
WHERE student1.sid=cj.sid 
group by student1.sid,student1.sname
having  COUNT(cj.cid)>=3;

--4、查询选修了'数学'并且及格的姓名，课程名，成绩

and : 结果集同时满足所有条件
or: 结果集只要满足其中一个条件即可

not 非一般情况跟 not like, not between..and not in 一起使用

SELECT student1.sname,cj.cid,cj.score FROM student1,cj
WHERE student1.sid=cj.sid 
GROUP BY student1.sname,cj.cid,cj.score
HAVING     cj.cid=2 AND cj.score>60;

--5、查询每个学生至少选修了2门看起来像'语'的课程的学生姓名。

like 看起来像模糊查询当知晓某个字段部分值

匹配符：% 匹配0或者多个任意字符

SELECT student1.sname,course.cname,cj.cid,COUNT(cj.cid) FROM student1,course,cj
WHERE student1.sid=cj.sid AND course.cid=cj.cid AND course.cname LIKE '%语'
GROUP BY student1.sname,course.cname,cj.cid
HAVING COUNT(cj.cid)>2;

--6、查询不及格的课程超过2门的学生编号和姓名

SELECT student1.sid,student1.sname,cj.score FROM student1,cj
WHERE student1.sid=cj.sid
HAVING score<60;

--7、查询选择的课程中"语文"的成绩比"数学"的成绩高的学生编号和姓名

单行子查询：子查询返回单行多列或者单行多列数据跟> < = >= <=一起使用

SELECT * FROM 表名 WHERE 字段名=(
查询1)

SELECT student1.sid,student1.sname 
FROM (student1, 
(SELECT * FROM cj WHERE cid=(
SELECT cid FROM course WHERE cname='语文'))AS temp1,
(SELECT * FROM cj WHERE cid=(
SELECT cid FROM course WHERE cname='数学'))AS temp2)
WHERE student1.sid = temp1.sid 
AND temp1.sid = temp2.sid 
AND temp1.score>temp2.score;

--8、查询没有选择看起来像“老”的老师的课程的学生编号和姓名

not like

distinct 去重复
select distinct 字段名 from 表名

as 临时表名/集名

-- SELECT student1.sid,student1.sname
-- FROM (student1,
(SELECT * FROM cj WHERE cid in(
-- SELECT cid FROM course WHERE course.cname NOT LIKE '%老'))AS temp3)
-- WHERE student1.sid=temp3.sid                 #sid和sname会有重复数据

-- select distinct 字段名 from 表名       #去重

SELECT DISTINCT student1.sid,student1.sname
FROM (student1,
(SELECT * FROM cj WHERE cid in(
SELECT cid FROM course WHERE course.cname NOT LIKE '%老'))AS temp3)
WHERE student1.sid=temp3.sid

--9、查询选修了课程的学员人数

-- SELECT COUNT(sid) FROM (
-- SELECT * FROM student1 WHERE sid  in(SELECT sid FROM cj))

SELECT COUNT(sid) FROM (
SELECT * FROM student1 WHERE sid  in(SELECT sid FROM cj))as temp4;

#Err] 1248 - Every derived table must have its own alias   每个派生出来的表都必须有一个自己的别名,所以在其后面加上 as temp4即可