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 | john@example.com | | 2 | bob@example.com | | 3 | john@example.com | +----+------------------+
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 | john@example.com | | 2 | bob@example.com | +----+------------------+
两种方法不太懂，t,p?
DELETE FROM Person WHERE Id NOT IN (SELECT Id FROM (SELECT MIN(Id) Id FROM Person GROUP BY Email) p);
# Write your MySQL query statement below SELECT Email FROM Person GROUP BY Email HAVING COUNT(*) > 1
方法1： join再删掉不需要的。
DELETE p2 FROM Person p1 JOIN Person p2 ON p2.Email = p1.Email WHERE p2.Id > p1.Id;
方法2： 不用join, 用where:
DELETE p2 FROM Person p1, Person p2 WHERE p1.Email = p2.Email AND p2.Id > p1.Id;
Leetcode 196. Delete duplicate Emails. (Database)
原文：http://www.cnblogs.com/lettuan/p/6291618.html