182. Duplicate Emails
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Write a SQL query to find all duplicate emails in a table named Person.
+----+---------+ | Id | Email | +----+---------+ | 1 | a@b.com | | 2 | c@d.com | | 3 | a@b.com | +----+---------+
For example, your query should return the following for the above table:
+---------+ | Email | +---------+ | a@b.com | +---------+
Note: All emails are in lowercase.
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1 # Write your MySQL query statement below 2 select distinct p1.Email from 3 Person p1,Person p2 4 where p1.Email=p2.Email 5 and p1.Id!=p2.Id
SQL SELECT DISTINCT 语句
本章讲解 SELECT DISTINCT 语句。
SQL SELECT DISTINCT 语句
在表中，可能会包含重复值。这并不成问题，不过，有时您也许希望仅仅列出不同（distinct）的值。
关键词 DISTINCT 用于返回唯一不同的值。
语法：
SELECT DISTINCT 列名称 FROM 表名称
使用 DISTINCT 关键词
如果要从 "Company" 列中选取所有的值，我们需要使用 SELECT 语句：
SELECT Company FROM Orders
"Orders"表：
| Company | OrderNumber |
|---|---|
| IBM | 3532 |
| 3wschool | 2356 |
| Apple | 4698 |
| 3wschool | 6953 |
结果：
| Company |
|---|
| IBM |
| 3wschool |
| Apple |
| 3wschool |
请注意，在结果集中，3wschool 被列出了两次。
如需从 Company" 列中仅选取唯一不同的值，我们需要使用 SELECT DISTINCT 语句：
SELECT DISTINCT Company FROM Orders
结果：
| Company |
|---|
| IBM |
| 3wschool |
| Apple |
现在，在结果集中，"3wschool" 仅被列出了一次。