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 | +----+------------------+
DELETE p1 FROM Person p1 INNER JOIN Person p2 WHERE p1.Email = p2.Email AND p1.Id > p2.Id;
这里p1和p2都是Person的别名。inner join等于join。所以p1 join p2应该是以下的结果：
------+------+-----------------+ |P1.Id| p2.Id|Email | +-----+------+-----------------+ | 1 | 1 | john@example.com | | 2 | 2 | bob@example.com | | 3 | 3 | john@example.com | +----+-------------------------+
然后p1.id>p2.id所以id为3的被删除了，但是因为p1和p2都是person事实上，只有一个id。为了理解，写成了两列。
补充一些关于sql语句的知识：
- sql表别名的用法：as
select * from kettleoutputtable a
where a.os =2 and storename = 'anzhi'
和
select * from kettleoutputtable as a
where a.os =2 and storename = 'anzhi'
等效，也就是说别名的as可以省略！在表明后直接加上简单的名字就行了。