Question

我在解决这个问题时遇到了一个难题：

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  |

例如，给定上面的Logs表，1是连续出现至少三次的唯一数字。

+-----------------+
| ConsecutiveNums |
+-----------------+
| 1               |
+-----------------+

我的第一个解决方案：

SELECT DISTINCT Num As ConsecutiveNums
FROM (
    SELECT 
    ID, 
    Num,
    @PreNum:=Num as PreNum,
    @Count:=IF(@PreNum=Num, @Count+1,1) AS Count

    FROM Logs,
    (SELECT @PreNum:=0,@Count:=1) x

    ) y
where y.Count>=3;

上面的代码将返回不正确的结果：

{"headers":["ConsecutiveNums"],"values":[[1],[2]]}

但是，如果我移动线＆＃34; @Count：= IF（@ PreNum = Num，@ Count + 1,1）AS Count，＆＃34;以上＆＃34; @PreNum：= Num作为PreNum＆＃34;，它将产生预期的结果。为什么这两行的顺序很重要？谢谢！

SELECT DISTINCT Num As ConsecutiveNums
FROM (
    SELECT 
    ID, 
    Num,
    @Count:=IF(@PreNum=Num, @Count+1,1) AS Count,
    @PreNum:=Num as PreNum


    FROM Logs,
    (SELECT @PreNum:=0,@Count:=1) x

    ) y
where y.Count>=3;

结果：

{"headers":["ConsecutiveNums"],"values":[[1]]}

Answer 1

我现在明白了：

@Count:=IF(@PreNum=Num, @Count+1,1) AS Count,
@PreNum:=Num as PreNum

如果我们将@Count留在＆＃34; @PreNum：= Num作为PreNum＆＃34; ，这意味着If逻辑仍在检查未在＆＃34; @PreNum：= Num作为PreNum＆＃34;重新分配的@PreNum。

MySQL连续数字问题：计数查询的顺序有关系吗？

1 个答案: