mysql查找刪除重復數據並只保留一條實例詳解

有這樣一張表，表數據及結果如下：

 

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

我們可以看出，school_name的字段值有重復數據(Abraham Lincoln High School 和Agoura High School分別出現兩次)，那么如何刪除這兩條數據，從而只讓這兩個數值出現一次呢？ 具體實現方法如下：

1、刪除重復記錄,保存Id最小的一條

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語法查詢出重復的數據，然后刪除重復數據並保留school_id最小的一條.

2、刪除重復記錄,保存Id最大的一條

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)

原理和上面一樣。