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196. Delete Duplicate Emails
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] | +----+------------------+
一开始我以为只是简单SELECT出来然后执行DELETE，这样居然不行，会报错“You can’t specify target table for update in FROM clause“，原因是在MYSQL中，不能先select一个表的记录，在按此条件进行更新和删除同一个表的记录。解决办法是，将select得到的结果，再通过中间表select一遍，这样就避免了错误，这个问题只出现于mysql，ms sql和oracle不会出现此问题。
# Write your MySQL query statement below
DELETE FROM Person
WHERE Id IN (SELECT a.Id FROM (
SELECT p1.Id FROM Person p1, Person p2
WHERE p1.Id > p2.Id AND p1.Email = p2.Email) AS a
);