Mysql中根据子表的分组后的最大统计数获取主表中的信息

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修改密码 ：mysqladmin  -u用户名 -p旧密码 password 新密码

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         ...................还有简单的数据库操作不做例举

下面尝试一些数据库中相对比较复杂的查询语句。

案例1：我想获取某一个学生的全部信息，该学生的发帖数量最多。学生与帖子是俩个表，帖子关联学生的主键stu_id;

俩张表中的内容如下：

mysql> select pp_id,pp_title,stu_id from tb_postpage;

+-------+-----------+--------+

| pp_id | pp_title  | stu_id |

+-------+-----------+--------+

|     1 | sdfasf    |      1 |

|     2 | 一起啊    |      1 |

|     3 | sfsdg     |      1 |

|     4 | jiayia    |      2 |

|     6 | dasfasf   |      2 |

|     7 | fasdfasdf |      3 |

|     8 |           |      3 |

+-------+-----------+--------+

7 rows in set

mysql> select stu_id,stu_count,stu_password from tb_student;

+--------+-----------+--------------+

| stu_id | stu_count | stu_password |

+--------+-----------+--------------+

|      1 | 123456    | 123456       |

|      2 | 140950105 | 051315       |

|      3 | 13145     | pangjiafnei  |

+--------+-----------+--------------+

1.先要对tb_postpage做一个分组。

  select * from tb_postpage group by stu_id;

   select pp_id,pp_title,stu_id from tb_postpage group by stu_id;

+-------+-----------+--------+

| pp_id | pp_title  | stu_id |

+-------+-----------+--------+

|     1 | sdfasf    |      1 |

|     4 | jiayia    |      2 |

|     7 | fasdfasdf |      3 |

+-------+-----------+--------+

3 rows in set

2.获得每组分组之后的计数以及stu_id

mysql> select stu_id,count(*) as count from tb_postpage group by stu_id;

+--------+-------+

| stu_id | count |

+--------+-------+

|      1 |     3 |

|      2 |     2 |

|      3 |     2 |

+--------+-------+

3 rows in set

3.获取count中的最大值

mysql> select stu_id,max(count) from (select stu_id,count(*) as count from tb_postpage group by stu_id) aa;

+--------+------------+

| stu_id | max(count) |

+--------+------------+

|      1 |          3 |

+--------+------------+

1 row in set【注意，这里要为表起一个别名】

4.获取该表中的stu_id;

mysql> select stu_id from (select stu_id,max(count) from (select stu_id,count(*) as count from tb_postpage group by stu_id) aa) bb;

+--------+

| stu_id |

+--------+

|      1 |

+--------+

1 row in set

5.通过该id获取学生的全部信息

mysql> select stu_id,stu_count,stu_password from tb_student where stu_id = (select stu_id from (select stu_id,max(count) from (select stu_id,count(*) as count from tb_postpage group by stu_id) aa) bb);

+--------+-----------+--------------+

| stu_id | stu_count | stu_password |

+--------+-----------+--------------+

|      1 | 123456    | 123456       |

+--------+-----------+--------------+

1 row in set