1. select 多子句单表高级实践

1.1 select 多子句高级语法

1. where 和 having 区别是后者是分组后进行排名
2. select 语法: select <字段1, 字段2, ...> from <表名> [where 条件]
3. 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

1. 什么是聚合函数: 聚合函数是group by 的前提条件

2. 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;