表結構
Student(s_id, sname, sage, ssex) 學生表
Course(c_id, cname, t_id)課程表
SC(s_id, c_id, score)成績表
Teacher(t_id,tname)教師表
建表語句
CREATE TABLE `student` (
`s_id` int(11) DEFAULT NULL,
`sname` varchar(32) DEFAULT NULL,
`sage` int(11) DEFAULT NULL,
`ssex` varchar(8) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
CREATE TABLE `course` (
`c_id` int(11) DEFAULT NULL,
`cname` varchar(32) DEFAULT NULL,
`t_id` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
CREATE TABLE `sc` (
`s_id` int(11) DEFAULT NULL,
`c_id` int(11) DEFAULT NULL,
`score` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
CREATE TABLE `teacher` (
`t_id` int(11) DEFAULT NULL,
`tname` varchar(16) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
面試問題
1. select的結果可以當做一個表
查詢“001”課程比“002”課程成績高的所有學生的學號;
select a.s_id from (select s_id,score from SC where c_id='001') a,(select s_id,score
from SC where c_id='002') b
where a.score>b.score and a.s_id=b.s_id;
2. 聚集函數和groupby一起出現,where不能連用
查詢平均成績大於60分的同學的學號和平均成績;
select s_id,avg(score)
from sc
group by s_id having avg(score) >60;
3. 連接查詢+groupby
查詢所有同學的學號、姓名、選課數、總成績
select s.s_id, s.sname, count(c.c_id), sum(c.score)
from student s, sc c where s.s_id = c.s_id
group by s.s_id;
4. 子查詢 in 、not in
查詢沒學過“葉平”老師課的同學的學號、姓名;
select Student.S#,Student.Sname
from Student
where s_id not in (
select distinct( SC.S_id) from SC,Course,Teacher
where SC.c_id=Course.c_id and
Teacher.t_id=Course.t_id and Teacher.Tname='葉平');
查詢至少有一門課與學號爲“1001”的同學所學相同的同學的學號和姓名;
select distinct s_ic,sname from Student,SC where Student.s_id=SC.s_id and SC.c_id in (select c_id from SC where s_id='1001');
5. and 不能連接同一個字段
查詢學過1並且也學過編號2課程的同學的學號、姓名;
正確寫法:
select s_id from sc where score = 90 and c_id in (1,2);
錯誤寫法:
select s_id from sc where score = 90 and c_id = 1 and c_id = 2;
6. 查詢同名同性學生名單,並統計同名人數
select sname,count(*) from Student group by sname having count(*)>1;
7. Order by 多個字段
例如order by id, score desc
首先會按照id降序排列,當id相同時,再按score降序排列
查詢每門課程的平均成績,結果按平均成績升序排列,平均成績相同時,按課程號降序排列
select c_id, avg(score) from sc GROUP BY c_id order by avg(score) , c_id desc;
8. group by多個字段
例如group by s_id, c_id
表示屬於s_id, 又屬於c_id的,例如屬於1號學生的,又屬於2號課程的
工作流程:
首先按照s_id分組,分組的結果再用c_id來分組
查詢平均成績大於85的所有學生的姓名和平均成績;
select s.s_id, s.sname, avg(c.score) from student s, sc c
where s.s_id = c.s_id group by s.sname , s.sage having avg(score) > 80;
因爲學生可能同名,所以group by s.sname , s.sage的作用就是,先按姓名分組,要是有重複的姓名,再按照性別分組。
9. MySQL不支持top,用limit,而且limit不能用於子查詢
查詢每門功成績最好的前兩名
錯誤寫法:
select s.s_id, s.sname , c.score from student s, sc c
where s.s_id = c.s_id and score in(
select score from sc GROUP BY s_id order by score desc limit 2);
正確寫法:
select s.s_id, s.sname , c.score from student s, sc c
where s.s_id = c.s_id and score in(
select score from sc GROUP BY s_id order by score desc)
limit 2;