題目：
Write a SQL query to delete all duplicate email entries in a table named Person,
keeping only unique emails based on its smallest Id.
+----+------------------+ | Id | Email | +----+------------------+ | 1 | [email protected] | | 2 | [email protected] | | 3 | [email protected] | +----+------------------+ Id is the primary key column for this table.
For example, after running your query, the above Person table
should have the following rows:
+----+------------------+ | Id | Email | +----+------------------+ | 1 | [email protected] | | 2 | [email protected] | +----+------------------+
select * from Person group by Email;原表：
查詢結果：
第二次嘗試代碼：
select * from Person group by Email order by Id;在自己的機器上通過，結果在Leetcode上報錯。
剛開始還以爲是版本的問題，我的是5.7，Leetcode是5.5。
後來仔細審題才發現，是自己審題出錯了。題目要求寫SQL語句刪除Person表中的重複數據（Write a SQL query
to delete all duplicate email entries in a table named
Person, ），而我之前所做的僅僅是查詢而已。第三次嘗試代碼：
delete p1 from Person p1, Person p2 where p1.Email=p2.Email and p1.Id>p2.Id;只要刪除內部的重複Email即可，使用內連接，加p1.Email=p2.Email和p1.Id>p2.Id兩個條件，即可刪除p1中的重複記錄。
驗證通過。
本題知識點：
（1）刪和查的概念不要搞混；
（2）內連接。