发布于 2017-08-16 01:46:17 | 163 次阅读 | 评论: 0 | 来源: 网友投递
Mysql关系型数据库管理系统
MySQL是一个开放源码的小型关联式数据库管理系统，开发者为瑞典MySQL AB公司。MySQL被广泛地应用在Internet上的中小型网站中。由于其体积小、速度快、总体拥有成本低，尤其是开放源码这一特点，许多中小型网站为了降低网站总体拥有成本而选择了MySQL作为网站数据库。
有这样一张表，表数据及结果如下：
| school_id | school_name | total_student | test_takers |
| 1239 | Abraham Lincoln High School | 55 | 50 |
| 1240 | Abraham Lincoln High School | 70 | 35 |
| 1241 | Acalanes High School | 120 | 89 |
| 1242 | Academy Of The Canyons | 30 | 30 |
| 1243 | Agoura High School | 89 | 40 |
| 1244 | Agoura High School | 100 | 50 |
我们可以看出，school_name的字段值有重复数据(Abraham Lincoln High School 和Agoura High School分别出现两次)，那么如何删除这两条数据，从而只让这两个数值出现一次呢？ 具体实现方法如下：
1、删除重复记录,保存Id最小的一条
delete FROM `test` WHERE `school_name` in (SELECT `school_name`
FROM `test`
GROUP BY `school_name`
HAVING COUNT( * ) >1) and school_id not in (select min(school_id) from test group by school_id having count(* )>1)
先使用GROUP BY having语法查询出重复的数据，然后删除重复数据并保留school_id最小的一条.
2、删除重复记录,保存Id最大的一条
delete FROM `test` WHERE `school_name` in (SELECT `school_name`
FROM `test`
GROUP BY `school_name`
HAVING COUNT( * ) >1) and school_id not in (select max(school_id) from test group by school_id having count(* )>1)
原理和上面一样。
以上就是mysql查找删除重复数据并只保留一条实例详解，希望能帮助到大家，谢谢大家对本站的支持！