1. select 多子句单表高级实践

1.1 select 多子句高级语法

where 和 having 区别是后者是分组后进行排名
select 语法: select <字段1, 字段2, ...> from <表名> [where 条件]
其他语句
1. group by 分组条件: 分组对指定列分组
2. having 条件: 分组后条件判断或过滤
3. order {col_name | expr | position} [ASC | DESC]: 排序ASC升序, DESC降序
4. LIMIT {[offset,] row_count | row_count OFFSET offset}: 限制结果集数量

1.2 聚合函数

什么是聚合函数: 聚合函数是group by 的前提条件

常用聚合函数

序号	聚合函数	用处
1	count()	返回指定组中数据的数量, 括号内加列名
2	sum()	返回指定组中数据之和, 只能用于数字列
3	avg()	返回指定组中的平均值
4	max()	返回指定数据的最大值
5	min()	返回指定数据的最小值
6	group_concat()	返回指定的数据, 按逗号分割为一行

1.3 group by 实践

查看表结构

MariaDB [world]> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   |     |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+

统计每个国家的总数

错误

select countrycode,sum(population) 
from city;

正确

select countrycode,sum(population) 
from city 
group by countrycode;

统计中国每个省的城市个数

SELECT District,COUNT(ID)
	FROM city 
	WHERE CountryCode='chn'
	GROUP BY District;

统计中国每个省的人口数

SELECT District,SUM(Population)
	FROM city 
	WHERE CountryCode='chn'
	GROUP BY District;

1.4 having 筛选

用途: 在group by 之后进行筛选

例子

统计中国每个省的城市个数以及城市列表,并把超过十个城市的输出

SELECT district,COUNT(*),GROUP_CONCAT(NAME) 
  FROM city 
  WHERE countrycode='CHN' GROUP BY district 
  having count(*)>10;

1.5 order by 排序

查询中国城市人口并按人口数排序输出

SELECT NAME,Population
	FROM city 
	WHERE CountryCode= 'CHN'
	ORDER BY Population DESC;

统计中国每个省的总人口，过滤输出总人口超过1000w,从大到小排序输出

SELECT District,SUM(Population)
	FROM city 
	WHERE CountryCode= 'CHN'
	GROUP BY District
	HAVING SUM(Population)>10000000
	ORDER BY SUM(Population) DESC;

1.6 limit

作用与语法: 用于显示指定数据行数, 一般用于order by排序后, 例如选择top3 或者倒数前3
例子
- 显示前两行: limit 2
- 显示3-6行: LIMIT 2,5

2. 多表连接

什么是多表连接: 将多张表用一个相同字段拼接成一张表
连接的分类
语法: select 字段1,字段2 from 表1 join 表2 on(注意此时的字段表示方法都应该是: 表.字段)
练习请参考: https://www.cnblogs.com/oldboy666/p/15637461.html

例子

查询oldboy老师教的学生名

SELECT teacher.tname,GROUP_CONCAT(student.sname)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc 
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE teacher.tname='oldboy'
GROUP BY teacher.tno;

查询oldboy所教课程的平均分数

SELECT teacher.tname,AVG(sc.score) 
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc 
ON course.cno=sc.cno
WHERE teacher.tname='oldboy'
GROUP BY teacher.tno;

查询oldboy老师教的学生名

SELECT teacher.tname,GROUP_CONCAT(student.sname)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc 
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE teacher.tname='oldboy'
GROUP BY teacher.tno;