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 | +-----------------+
Write a SQL query to find all numbers that appear at least three times consecutively. For example, given the above Logs table, 1 is the only number that appears consecutively for at least three times.
分析：题目要求连续出现至少三次，比较容易想到的是使用三个表实例，利用ID自增连续的特点，返回满足条件的Num
select distinct l1.Num ConsecutiveNums from Logs l1 left join Logs l2 on l1.Id = l2.Id - 1 left join Logs l3 on l1.Id = l3.Id - 2 where l1.Num = l2.Num and l2.Num = l3.Num; or select distinct l1.Num ConsecutiveNums from Logs l1,Logs l2,Logs l3 where l1.Id = l2.Id-1 and l1.Id = l3.Id-2 and l1.Num = l2.Num and l2.Num = l3.Num;另解：
SELECT num as ConsecutiveNums from ( SELECT *,@rank := if(@num = num ,@rank + 1 ,1 ) AS num2, @num:=num as num3 from logs, (SELECT @num := 0,@rank := 0) tt )ttt where num2>2 group by num
