【leetcode Database】180. Consecutive Numbers

題目：

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.

解析：

首先，增加一個rank字段，記錄序號，初始值爲1，當後一個值與前一個值相等時，序號加1。之後，把所有rank值大於等於3的都檢索出來，再去重即可。代碼如下：

# Write your MySQL query statement below
 SELECT DISTINCT(Num) AS ConsecutiveNums FROM 
 (SELECT Id,Num, 
 @Rank:=IF(@prevNum != Num,1,@Rank+1) AS Rank,
 @prevNum:=Num
 FROM Logs) t,
 (SELECT @Rank:=0,@prevNum:=NULL) r
 WHERE t.Rank >= 3;