我正在编写代码以获得滞后时间和提前期。我能够成功地获得正确的超前值，但是，不正确的滞后值。代码如下：

Select t1.user_id, t1.time_period,

t1.time_period - (select t2.time_period from monthly_usage as t2
                         where t2.user_id = t1.user_id and 
                         t2.time_period < t1.time_period
                         order by t1.time_period desc LIMIT 1) as lag,
(SELECT 
            t2.time_period
        FROM
            monthly_usage as t2
        WHERE
            t2.user_id = t1.user_id
                AND t2.time_period > t1.time_period
        ORDER by t1.time_period
        LIMIT 1) AS lead

from monthly_usage as t1;

我的输出是这样的

但我希望输出如下。

我的代码有错误吗？