[Medium]Consecutive Numbers

Write a SQL query to find all numbers that appear at least three times consecutively.
+——+——-+
| 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.
+————————-+
| ConsecutiveNums |
+————————-+
| 1 |
+————————-+

Hint:建立3表连接，用a.id=b.id-1得到这样的结构，再去验证3张表的num是否一样

Answer：