LeetCode笔记：（MySQL） 182. Duplicate Emails

Write a SQL query to find all duplicate emails in a table named Person.

+----+---------+
| Id | Email   |
+----+---------+
| 1  | [email protected] |
| 2  | [email protected] |
| 3  | [email protected] |
+----+---------+

For example, your query should return the following for the above table:

+---------+
| Email   |
+---------+
| [email protected] |
+---------+

Note: All emails are in lowercase.

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My Solution:

SELECT DISTINCT
    person1.Email
FROM
    person person1
INNER JOIN person person2 ON person1.email = person2.email
WHERE
    person1.id <> person2.id

思路：

先用inner join拼接Person表中含有相同Email的所有id，然后再使用DISTINCT关键字去除重复的记录。（性能较低，倒数12%）

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Other People’s Solutions:

SELECT
    Email
FROM
    Person
GROUP BY
    Email
HAVING
    count(*) > 1

摘自 —— https://discuss.leetcode.com/topic/7485/i-have-this-simple-approach-anybody-has-some-other-way
作者：dashinghimay

OR

SELECT DISTINCT
    Email
FROM
    Person
HAVING
    count(*) > 1

思路：

先查询出所有不重复的Email，再使用count()函数对结果进行筛选，最终筛选出次数大于一次的所有Email。

注意：

（使用GROUP BY可以限制查询结果中某一个字段不重复，而使用DISTINCT则为限制查询结果中所有字段同时不重复）
例：

 1.此处查询的是Id及Email字段，可以仅限制Email字段不重复。

SELECT Id, Email
FROM Person
GROUP BY Email DESC

 2.此处查询的也是Id及Email字段，但限制为Id及Email字段同时不重复。

SELECT DISTINCT Id, Email
FROM Person

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拓展：

HAVING 子句
在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与合计函数一起使用。

SQL HAVING 子句 —— W3School
http://www.w3school.com.cn/sql/sql_having.asp