在 MySQL 的資料庫中，若有些欄位我們在使用後才想要讓某個欄位的值不重複，讓如何刪除這個欄位中重複的值呢？例如我們在開發會員系統時，資料建置後才想到要讓電子郵件欄位不重複，就會遇到這個問題。
以下我們以一個 people 資料表來進行說明：
如果我們想找尋資料表中多餘的重複記錄，重複記錄是根據單個字段（peopleId）來判斷：
select * from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1) 刪除表中多餘的重複記錄，重複記錄是根據單個字段（peopleId）來判斷，只留有 rowid 最小的記錄
delete from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1) and rowid not in (select min(rowid) from people group by peopleId having count(peopleId )>1) 尋找資料表中多餘的重複記錄（多個字段）
delete from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1) and rowid not in (select min(rowid) from people group by peopleId having count(peopleId )>1) 刪除資料表中多餘的重複記錄（多個字段），只留有 rowid 最小的記錄
delete from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1) and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1) 尋找資料表中多餘的重複記錄（多個字段），不包含rowid最小的記錄
select * from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1) and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1) 提供你參考。