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 | 120 | |
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。