求助，下一条减上一条 得出结果，求助

现取得以下数据：
时间                                    当前水位                     昨日蒸发                    昨日雨量
2015-4-4 8:00:00                       31.9                                 0.5                             41.3
2015-4-5 8:00:00                30.0                                 0.7                             63.3
2015-4-6 8:00:00                18.1                                 0.5                             24.4
2015-4-7 8:00:00                28.9                                 0.9                             33.5
2015-4-8 8:00:00                30.1                          1.0                             2.5
2015-4-9 8:00:00                28.9                          1.5                             0.0
2015-4-10 8:00:00               27.8                          1.2                             0.0
2015-4-11 8:00:00               26.4                          1.6                             0.0
2015-4-12 8:00:00               24.5                          2.3                             0.0
2015-4-13 8:00:00               21.3                          3.9                             0.0
2015-4-14 8:00:00               17.9                          4.2                             0.0
2015-4-15 8:00:00               15.0                                 3.6                             0.0SQL数据库，使用ASP，用下一条数据减上一条数据求差值。怎么计算呢？
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