MySQL数据库之多表查询using优化与案例

using

• 概念
  • using用来指定连接字段
  • using的结果也会对公共字段进行优化，优化的规则和自然连接是一样的

MariaDB [sel]> select * from grades inner join resume using(name);
+-------+---------+------+----+-----------+
| name  | chinese | math | id | skill     |
+-------+---------+------+----+-----------+
| Sunny |      93 |   96 |  1 | php       |
| Jerry |      97 |   91 |  3 | php,mysql |
+-------+---------+------+----+-----------+
# `2 rows in set (0.001 sec)`

MySQL练习题

显示地区及每个地区参加笔试的人数，并按人数降序排列

• 思路分解
  • select 查询字段
  • from 多表查询 左外连接
  • using 指定连接字段
  • group by 分组查询结果
  • order by 降序排列

-- 第一步： 显示地区及每个地区参加笔试的人数
mysql> select stuaddress,count(writtenexam) from stuinfo left join stumarks using(stuno) group by stuaddress;
+------------+--------------------+
| stuaddress | count(writtenexam) |
+------------+--------------------+
| 上海       |                  1 |
| 北京       |                  2 |
| 天津       |                  2 |
| 河北       |                  0 |
| 河南       |                  0 |
+------------+--------------------+
# `5 rows in set (0.00 sec)`

-- 第二步：将结果降序排列
mysql> select stuaddress,count(writtenexam) c from stuinfo left join stumarks using(stuno) group by stuaddress order by c desc;
+------------+---+
| stuaddress | c |
+------------+---+
| 北京       | 2 |
| 天津       | 2 |
| 上海       | 1 |
| 河北       | 0 |
| 河南       | 0 |
+------------+---+
# `5 rows in set (0.00 sec)`

显示有学生参加考试的地区

• 思路解析
  • select 选择查询字段
  • from 多表查询 左外连接
  • using 指定连接字段
  • group by 分组查询显示
  • having 条件筛选

-- having筛选
mysql> select stuaddress,count(writtenexam) c from stuinfo left join stumarks using(stuno) group by stuaddress having c>0;
+------------+---+
| stuaddress | c |
+------------+---+
| 上海       | 1 |
| 北京       | 2 |
| 天津       | 2 |
+------------+---+
# `3 rows in set (0.00 sec)`

• 思路解析
  • select 选择查询字段
  • from 多表查询 右外连接
  • using 指定连接字段
  • distinct 去重复
  • having 条件筛选
  • is not null 去空

-- 表连接实现
-- 第一步：右连接获取有成绩的地区
mysql> select stuaddress from stuinfo right join stumarks using(stuno);
+------------+
| stuaddress |
+------------+
| 北京       |
| 上海       |
| 天津       |
| 北京       |
| 天津       |
| NULL       |
+------------+
# `6 rows in set (0.00 sec)`

-- 第二步：去重复
mysql> select distinct stuaddress from stuinfo right join stumarks using(stuno);
+------------+
| stuaddress |
+------------+
| 北京       |
| 上海       |
| 天津       |
| NULL       |
+------------+
# `4 rows in set (0.00 sec)`

-- 去除null
mysql> select distinct stuaddress from stuinfo right join stumarks using(stuno) having stuaddress is not null;
+------------+
| stuaddress |
+------------+
| 北京       |
| 上海       |
| 天津       |
+------------+
# `3 rows in set (0.00 sec)`

显示男生和女生的人数

• 方法一:分组查询
  • select 查询字段
  • from 查询表
  • group by 分组查询显示

mysql> select stusex,count(*) from stuinfo group by stusex;
+--------+----------+
| stusex | count(*) |
+--------+----------+
| 女     |        3 |
| 男     |        4 |
+--------+----------+
# `2 rows in set (0.00 sec)`

• 方法二:union
  • select 查询字段
  • from 查询表
  • where 条件筛选
  • union 联合查询

mysql> select stusex,count(*) from stuinfo where stusex='男' union select stusex,count(*) from stuinfo where stusex='女';
+--------+----------+
| stusex | count(*) |
+--------+----------+
| 男     |        4 |
| 女     |        3 |
+--------+----------+
# `2 rows in set (0.00 sec)`

• 方法三:直接写条件
  • select 聚合函数查询
  • from 查询表

mysql> select sum(stusex='男') 男,sum(stusex='女') 女 from stuinfo;
+------+------+
| 男   | 女   |
+------+------+
|    4 |    3 |
+------+------+
# `1 row in set (0.00 sec)`

显示每个地区男生、女生、总人数

• 思路解析
  • select 选择字段 聚合函数
  • from 选择表
  • group by 分组查询显示

mysql> select stuaddress,count(*) 总人数,sum(stusex='男') 男,sum(stusex='女') 女 from stuinfo group by stuaddress;
+------------+--------+------+------+
| stuaddress | 总人数 | 男    | 女   |
+------------+--------+------+------+
| 上海       |      1 |    1 |    0 |
| 北京       |      2 |    1 |    1 |
| 天津       |      2 |    2 |    0 |
| 河北       |      1 |    0 |    1 |
| 河南       |      1 |    0 |    1 |
+------------+--------+------+------+
# `5 rows in set (0.00 sec)`