mysql学习笔记-day2

Posted 2020-07-18

1、查询“001”课程比“002”课程成绩高的所有学生的学号；
select a.`S#` from (select `s#`,score from SC where `C#`="001") a,
(select `s#`,score from SC where `C#`="002") b
where a.score>b.score and a.`s#`=b.`s#`;

2、查询平均成绩大于60分的同学的学号和平均成绩；
select `S#`,avg(score)
from sc
group by `S#` having avg(score) >60;

3、查询所有同学的学号、姓名、选课数、总成绩；
select Student.`S#`,Student.Sname,count(SC.`C#`),sum(score)
from Student left Outer join SC on Student.`S#`=SC.`S#`
group by Student.`S#`,Sname ;

4、查询姓“李”的老师的个数；
select count(distinct(Tname)) from Teacher where Tname like "李%";

5、查询没学过“叶平”老师课的同学的学号、姓名；
select Student.`S#`,Student.Sname
from Student
where `S#` not in (select distinct( SC.`S#`) from SC,Course,Teacher where SC.`C#`=Course.`C#` and Teacher.`T#`=Course.`T#` and Teacher.Tname="叶平");

6、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；
select Student.`S#`,Student.Sname
from Student,SC
where Student.`S#`=SC.`S#` and SC.`C#`="001"and exists( Select * from SC as SC_2 where SC_2.`S#`=SC.`S#` and SC_2.`C#`="002");

7、查询学过“叶平”老师所教的所有课的同学的学号、姓名；
select `S#`,Sname from Student where `S#` in (select `S#`from SC ,Course ,Teacher
where SC.`C#`=Course.`C#` and Teacher.`T#`=Course.`T#` and Teacher.Tname="叶平" group by `S#`
having count(SC.`C#`)=(select count(`C#`) from Course,Teacher where Teacher.`T#`=Course.`T#` and Tname="叶平"));

8、查询所有课程成绩小于60分的同学的学号、姓名；
select `S#`,Sname from Student
where `S#` not in (select Student.`S#` from Student,SC where Student.`S#`=SC.`S#` and score>60);

9、查询没有学全所有课的同学的学号、姓名；
select Student.`S#`,Student.Sname from Student,SC where Student.`S#`=SC.`S#`
group by Student.`S#`,Student.Sname having count(`C#`) <(select count(`C#`) from Course);

10、更改表字段类型

alter table student modify `s#` varchar(15);

PS.今天面试回来，完全蒙圈的感觉，四年的工作经验仅仅价值不到1000，好吧，行业或许真的很重要，要选择一个行业，坚持走下去，而不是再换行业或者半途而废了。

走到无路可走，还是要坚持走下去，一个人又如何？谁不是自己杀出一条血路，然后才有了陪伴的人？又或许，现在也会有一类人，根本无须陪伴，只是要孤军奋战。