当前位置： 修改数据库最大连接数-贵阳大数据数博会2018
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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修改数据库最大连接数，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【注意修改数据库最大连接数，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
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