MySql 50句及解析

根據互聯網資源二次加工得來

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一 創建語句

create table Student(sid varchar(10),sname varchar(10),sage datetime,ssex nvarchar(10));
insert into Student values('01' , '趙雷' , '1990-01-01' , '男');
insert into Student values('02' , '錢電' , '1990-12-21' , '男');
insert into Student values('03' , '孫風' , '1990-05-20' , '男');
insert into Student values('04' , '李雲' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吳蘭' , '1992-03-01' , '女');
insert into Student values('07' , '鄭竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');
create table Course(cid varchar(10),cname varchar(10),tid varchar(10));
insert into Course values('01' , '語文' , '02');
insert into Course values('02' , '數學' , '01');
insert into Course values('03' , '英語' , '03');
create table Teacher(tid varchar(10),tname varchar(10));
insert into Teacher values('01' , '張三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');
create table SC(sid varchar(10),cid varchar(10),score decimal(18,1));
insert into SC values('01' , '01' , 80);
insert into SC values('01' , '02' , 90);
insert into SC values('01' , '03' , 99);
insert into SC values('02' , '01' , 70);
insert into SC values('02' , '02' , 60);
insert into SC values('02' , '03' , 80);
insert into SC values('03' , '01' , 80);
insert into SC values('03' , '02' , 80);
insert into SC values('03' , '03' , 80);
insert into SC values('04' , '01' , 50);
insert into SC values('04' , '02' , 30);
insert into SC values('04' , '03' , 20);
insert into SC values('05' , '01' , 76);
insert into SC values('05' , '02' , 87);
insert into SC values('06' , '01' , 31);
insert into SC values('06' , '03' , 34);
insert into SC values('07' , '02' , 89);
insert into SC values('07' , '03' , 98);

約定 student s ，course c ，teacher t，sc不動

二 50句

查詢

1.查詢同一學生的課程1 比課程 2 成績高的所有學生的學號；
SELECT a.sid
FROM 
(SELECT sid,score FROM sc WHERE cid = 1)a,
(SELECT sid,score FROM sc WHERE cid = 2)b
WHERE a.score > b.score 
AND a.sid = b.sid;
先找出兩個課程的成績，再通過where語句控制是同一個學生的成績`

2.查詢平均成績大於60分的同學的學號和平均成績；
SELECT sid,AVG(score) avg 
FROM sc 
GROUP BY sid 
HAVING avg > 60;
直接用公式就可，比較簡單

3.查詢所有同學的學號、姓名、選課數、總成績
<方式一>
SELECT s.sid,s.sname,COUNT(DISTINCT cid),SUM(score)
FROM student s,sc
WHERE s.sid = sc.sid
GROUP BY sid;
<方式二>
SELECT s.sid,s.sname,COUNT(DISTINCT cid),SUM(score)
FROM student s
LEFT JOIN sc ON s.sid = sc.sid
GROUP BY sid ;
第二種是新的查詢方式，二者一個意思
group by 的字段不需要使用s.sid,原因是分組字段明確。

4.查詢姓“李”的老師的個數；
SELECT COUNT(tname) FROM teacher
WHERE tname like '李%'
直接用公式就可，比較簡單

5.查詢沒學過“張三”老師課的同學的學號、姓名；
SELECT sid,sname
FROM student
WHERE sid  NOT IN
(SELECT sc.sid FROM sc,course c,teacher t 
WHERE sc.cid = c.cid 
AND c.tid = t.tid
AND t.tname = '張三'
);
先查詢學過的，不在學過的集合中，不就是沒學過的

6.查詢學過“語文”並且也學過“數學”課程的同學的學號、姓名；
SELECT a.sid,a.sname FROM
(SELECT s.sid,s.sname FROM student s,sc,course c WHERE c.cname = '語文' AND s.sid = sc.sid AND sc.cid = c.cid)a,
(SELECT s.sid,s.sname FROM student s,sc,course c WHERE c.cname = '語文' AND s.sid = sc.sid AND sc.cid = c.cid)b
WHERE a.sid = b.sid;
比較簡單

7.查詢學過“張三”老師所教的所有課的同學的學號、姓名；
SELECT s.sid,s.sname
FROM student s
LEFT JOIN sc ON sc.sid = s.sid
LEFT JOIN course c ON c.cid=sc.cid
LEFT JOIN teacher t ON t.tid = c.tid
WHERE t.tname = '張三'
GROUP BY s.sid 
HAVING COUNT(sc.cid) = (SELECT COUNT(c.tid) FROM course c,teacher t WHERE c.tid = t.tid AND t.tname='張三');
這個語句有點難，需要先查出來張三老師共教了幾節課
只學了部分張三老師課的還不行，需要對學過張三老師所有課進行一個count，只有二者相等纔算符合題目

8.查詢所有課程有成績小於60分的同學的學號、姓名；
SELECT s.sid,s.sname,sc.cid FROM student s,sc 
WHERE sc.score<60 
AND sc.sid=s.sid;
這個簡單

9.查詢沒有學全所有課的同學的學號、姓名
SELECT s.sid,s.sname 
FROM student s,sc 
WHERE s.sid=sc.sid 
GROUP BY s.sid,s.sname
 HAVING count(sc.cid)<(SELECT count(cid) from course);
和7有點類似