180. 连续出现的数字

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               |
+-----------------+

思路：与两个表进行SELF JOIN，l1和l2的Id下一个位置比，l1和l3的下两个位置比，然后将Num都相同的数字返回即可

SELECT DISTINCT l1.Num AS ConsecutiveNums 
FROM Logs AS l1, Logs AS l2, Logs AS l3
WHERE l2.Id=l1.Id+1 AND l3.Id=l2.Id+1 AND l2.Num=l1.Num AND l3.Num=l2.Num