mysql查询之 连续出现的数字，重复出现的邮箱,删除重复的电子邮箱

1.编写一个 SQL 查询，查找所有至少连续出现三次的数字。

+----+-----+
| Id | Num |
+----+-----+
| 1  |  1  |
| 2  |  1  |
| 3  |  1  |
| 4  |  2  |
| 5  |  1  |
| 6  |  2  |
| 7  |  2  |
+----+-----+

例如，给定上面的 Logs 表， 1 是唯一连续出现至少三次的数字。

+-----------------+
| ConsecutiveNums |
+-----------------+
| 1               |
+-----------------+

select Num as ConsecutiveNums  from Logs where Id in (
    select distinct l1.Id from Logs l1,Logs l2,Logs l3
    where (l1.Num = l2.Num and l2.Num = l3.Num and ( 
         (l1.Id + 1= l2.Id  and l2.Id +1 = l3.Id)or
         (l3.Id + 1= l2.Id  and l2.Id +1 = l1.Id)or
         (l3.Id + 1= l1.Id  and l1.Id +1 = l2.Id)
      )
    ) 
order by l1.Id)  group by Num

 

2.编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

示例：

+----+---------+
| Id | Email   |
+----+---------+
| 1  | a@b.com |
| 2  | c@d.com |
| 3  | a@b.com |
+----+---------+

根据以上输入，你的查询应返回以下结果：

+---------+
| Email   |
+---------+
| a@b.com |
+---------+

select Email from Person group by Email having count(Email)>1

 3.编写一个 SQL 查询，来删除 Person 表中所有重复的电子邮箱，重复的邮箱里只保留 Id 最小 的那个。

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+
Id 是这个表的主键

delete p1 from Person p1,Person p2 where p1.Email=p2.Email and p1.Id>p2.Id