4、查询（2）

1、分组聚合

聚合函数（组函数）：

• count　　个数
• max         最大值
• min　　  最小值
• sum         和
• avg           平均值

select 聚合函数 ，字段  where  [ exp] group by 字段 having 聚合函数条件

count（*）：

mysql> select count(*) from score where number>75;　　#满足条件的行数，没有条件则统计所有行数
+----------+
| count(*) |
+----------+
|        2 |
+----------+

count（列字段）：

mysql> select count(student_id) as stu,number from score group by number;
+-----+--------+
| stu | number |
+-----+--------+
|   1 |     68 |
|   1 |     75 |
|   1 |     80 |
|   1 |     90 |
+-----+--------+

count（distinct 列字段）：非空，不重复行数

mysql> select count(distinct student_id) from score ;
+----------------------------+
| count(distinct student_id) |
+----------------------------+
|                          3 |
+----------------------------+

min（字段），max（字段）：

mysql> select max(number) from score ;
+-------------+
| max(number) |
+-------------+
|          90 |
+-------------+

mysql> select min(number) from score ;
+-------------+
| min(number) |
+-------------+
|          68 |
+-------------+

avg（字段），sum（字段）：

mysql> select sum(number) from score where number>80;
+-------------+
| sum(number) |
+-------------+
|          90 |
+-------------+

mysql> select avg(number) from score ;
+-------------+
| avg(number) |
+-------------+
|     78.2500 |
+-------------+

group：（重要）

mysql> select name,count(*) from students group by id;
+--------+----------+
| name   | count(*) |
+--------+----------+
| 鲤鱼   |        1 |
| ee     |        1 |
| mike   |        1 |
| ldh    |        1 |
| res    |        1 |
+--------+----------+

2、连表（重要）

第一种：使用where

mysql> select * from score,class where score.student_id=class.cid;
+-----+------------+-----------+--------+-----+----------+
| sid | student_id | course_id | number | cid | caption  |
+-----+------------+-----------+--------+-----+----------+
|   1 |          3 |         2 |     68 |   3 | 材料三班 |
|   2 |          2 |         2 |     90 |   2 | 材料二班 |
|   3 |          1 |         3 |     75 |   1 | 材料一班 |
|   4 |          2 |         1 |     80 |   2 | 材料二班 |
|   5 |          1 |         3 |     80 |   1 | 材料一班 |
+-----+------------+-----------+--------+-----+----------+

第二种：left join 表  on   关系；left join左边全显示；right join右边全显示

mysql> select score.sid,sname,gender,cname,number,caption,tname from score 
　　　　　　left join student on score.student_id=student.sid 
　　　　　　left join course on score.course_id=course.cid 
　　　　　　left join class on class.cid=student.class_id 
　　　　　　left join teacher on teacher.tid=course.tearch_id;

+-----+-------+--------+-------+--------+----------+--------+
| sid | sname | gender | cname | number | caption  | tname  |
+-----+-------+--------+-------+--------+----------+--------+
|   3 | 章鱼  | 男     | 物理  |     75 | 材料二班 | 张老师 |
|   5 | 章鱼  | 男     | 物理  |     80 | 材料二班 | 张老师 |
|   2 | 李杰  | 女     | 大化  |     90 | 材料一班 | 张老师 |
|   4 | 李杰  | 女     | 高数  |     80 | 材料一班 | 任丘   |
|   1 | 吴雨  | 女     | 大化  |     68 | 材料二班 | 张老师 |
+-----+-------+--------+-------+--------+----------+--------+　　　　

第三种： inner join　　内连接求交集，不显示null的行

第四种：union　　上下连表

　　去重：select   查询内容  from 表名  UNION  select  查询内容  from  表名

　　不去重：... UNION  ALL ...

练习：1、检索‘物理’分数大于70，按分数降序排列的同学学号

mysql> select student.sid,sname,cname,number from score  
　　　　　　left join student on score.student_id=student.sid 
　　　　　　left join course on score.course_id=course.cid 
　　　　　　left join class on class.cid=student.class_id 
　　　　　　where number>70 and cname='物理' order by number;　　#条件放最后

+------+-------+-------+--------+
| sid  | sname | cname | number |
+------+-------+-------+--------+
|    1 | 章鱼  | 物理  |     75 |
|    1 | 章鱼  | 物理  |     80 |
+------+-------+-------+--------+

2、查询姓“李”的学生名单

mysql> select * from student;
+-----+--------+--------+----------+
| sid | sname  | gender | class_id |
+-----+--------+--------+----------+
|   1 | 章鱼   | 男     |        2 |
|   2 | 李杰   | 女     |        1 |
|   3 | 吴雨   | 女     |        2 |
|   4 | 李观星 | 男     |        2 |
+-----+--------+--------+----------+

mysql> select sname from student where sname like '李%';
+--------+
| sname  |
+--------+
| 李杰   |
| 李观星 |
+--------+

3、按性别分类

SELECT gender,COUNT(gender) as '人数' FROM student GROUP BY gender;

4、临时表，测试数据

SELECT * FROM (SELECT sid,sname FROM student) as b;　　#as b 必须要有

 5、查询平均成绩大于60分的同学的学号和平均成绩，姓名

SELECT student_id,sname,AVG(num) FROM score LEFT JOIN student ON student_id=student.sid GROUP BY student_id HAVING AVG(num)>60;
或
SELECT student_id,sname,aa FROM (SELECT *,AVG(num) as aa FROM score  GROUP BY student_id HAVING AVG(num)>60) as b
LEFT JOIN student ON student_id=student.sid;

6、没有选过刘老师的课的学生

SELECT student_id,sname FROM score LEFT JOIN student ON student.sid=student_id 
WHERE student_id not in (SELECT DISTINCT(student_id) FROM score WHERE course_id in (2,4))

7、生物成绩高于物理成绩

SELECT A.student_id,A.num,B.num FROM 
(SELECT * FROM score WHERE score.course_id = (SELECT cid FROM course WHERE cname='生物')) as A
INNER JOIN 
(SELECT * FROM score WHERE score.course_id = (SELECT cid FROM course WHERE cname='物理')) as B
ON A.student_id = B.student_id WHERE A.num > B.num

8、按平均成绩从低到高显示所有学生的“语文”、“数学”、“英语”三门的课程成绩，按如下形式显示： 学生ID,语文,数学,英语,有效课程数,有效平均分

SELECT student_id,
(SELECT num FROM score as s2 WHERE s1.student_id=s2.student_id and course_id=1) as 生物,
(SELECT num FROM score as s2 WHERE s1.student_id=s2.student_id and course_id=2) as 物理,
(SELECT num FROM score as s2 WHERE s1.student_id=s2.student_id and course_id=3) as 体育,
(SELECT num FROM score as s2 WHERE s1.student_id=s2.student_id and course_id=4) as 美术,
COUNT(1) as 有效课程数,AVG(num) as 平均分
 FROM score as s1 GROUP BY student_id ORDER BY 平均分 asc

9、查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；还能运算，判断

SELECT course_id,MAX(num),MIN(num),MIN(num)+1,CASE WHEN MIN(num)<10 THEN 0 ELSE MIN(num) END FROM score GROUP BY course_id

10、按各科平均成绩从低到高和及格率的百分数从高到低顺序

SELECT course_id,AVG(num),SUM(CASE WHEN num>60 THEN 1 ELSE 0 END),sum(1),SUM(CASE WHEN num>60 THEN 1 ELSE 0 END)/SUM(1) as jgl 
FROM score GROUP BY course_id ORDER BY AVG(num) ASC,jgl DESC

11、课程平均分从高到低显示（显示任课老师）

SELECT course_id,cname,AVG(num),tname FROM score 
LEFT JOIN course ON course.cid=course_id 
LEFT JOIN teacher on teacher.tid=course.teacher_id GROUP BY course_id ORDER BY AVG(num) DESC

或

SELECT course_id,cname,AVG(if(ISNULL(score.num),0,score.num)),tname FROM score 　　#if 语句相当于python三元表达式，目的是防止空的值计入计算
LEFT JOIN course ON course.cid=course_id 
LEFT JOIN teacher on teacher.tid=course.teacher_id GROUP BY course_id ORDER BY AVG(num) DESC