leetcode-database-180. 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.
题意：查询出连续出现3次的数字

思路：
定义参数preNum记录前一个Number的值，curRank记录目前连续出现的次数

@curRank:=@curRank +if(@preNum=Num,0,1)
如果当前的Number值等于preNum，那么curRank+1

select Num,@curRank:=@curRank + if(@preNum=Num,0,1) As rank,@preNum:=Num  from Logs s,(select @curRank:=0) r,(select @preNum:=NULL) p order by id ASC

+------+-------+

| Num | rank |

+------+-------+

| 1       |  1      |

| 1       |  1      |

| 1       |  1      |

| 2      |  2     |

| 1       |  3     |

| 2      |  4     |

| 2      |  4     |

+------+-------+

select Num,count(rank) as cnt
from(select Num,@curRank:=@curRank + if(@preNum=Num,0,1) As rank,@preNum:=Num  from Logs s,(select @curRank:=0) r,(select @preNum:=NULL) p order by id ASC) t

group by rank

+------+-------+

| Num | cnt    |

+------+-------+

| 1       |  3       |

| 2      |   1       |

| 1       |   1       |

| 2      |  2       |

+------+-------+

最终sql语句：

select distinct Num as ConsecutiveNums from (select Num,count(rank) as cnt
from(select Num,@curRank:=@curRank + if(@preNum=Num,0,1) As rank,@preNum:=Num  from Logs s,(select @curRank:=0) r,(select @preNum:=NULL) p order by id ASC) t

group by rank having cnt>=3) n;