Leetcode 196. Delete duplicate Emails. (Database)

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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)

标签：rom   mysq   john   cat   tab   trie   prim   ble   tco   

原文地址：http://www.cnblogs.com/lettuan/p/6291618.html