180. Consecutive Numbers（连续出现的数字）

题目：

Write a SQL query to find all numbers that appear at least three times consecutively.
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  |
+----+-----+

For example, given the above Logs table, 1 is the only number that appears consecutively for at least three times.
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               |
+-----------------+

解答思路：

注意到 Id 是从1开始递增的，则需要查找出 Id 连续且 Num 相等的数字即可，注意最后要重命名列名为 ConsecutiveNums。

select distinct l1.Num as ConsecutiveNums  
from  Logs l1, Logs l2, Logs l3   
where l1.Id=l2.Id-1 and l2.Id=l3.Id-1 
and l1.Num = l2.Num and l2.Num = l3.Num