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
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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，然后刪除重復數據并保留school_id最小的一條.
2、刪除重復記錄,保存Id最大的一條
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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。
