LeetCode - Delete Duplicate Emails
Discription：Write a SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its smallest Id.
删除重复的Email地址，保留Id最小的那个。
使用自身连接循环即可。
# Write your MySQL query statement below
delete p1 from Person p1, Person p2
where p1.Email=p2.Email and p1.Id>p2.Id;
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