mysql：比较同一表上的两行

Question

我有这样的输出：

+-----------+------------+--------------+
| client_id | date       | spends       |
+-----------+------------+--------------+
|        57 | 2014-09-28 | 39576.384391 |
|        57 | 2014-10-05 | 22664.382575 |
+-----------+------------+--------------+

我需要这样的行：如果前一周的支出为空，而本周有支出。

Answer 1

这是您需要的吗？

select t.*
from table t
where spend is not null and
      exists (select 1
              from table t2
              where t2.client_id = t.client and
                    t2.spends is null and
                    t2.date = t.date - interval 7 days
             );