MySQL的：获取上一行

Question

我有以下查询：

SELECT c.called as customer, c.calling as company, DATE_FORMAT(c.`end`,'%Y-%m-%d') as _date, 

     @G := if(@prevComp <> c.calling AND @prevCust = c.called AND @prevDay = DATE_FORMAT(c.`end`,'%Y-%m-%d'), 1, 0) as Gain, 
     @prevComp := c.called as prevComp, 
     @prevCust := c.calling as prevCust, 
     @prevDay := DATE_FORMAT(c.`end`,'%Y-%m-%d') as prevDate 

FROM cdrdata_archive c, 
    (SELECT @prevComp := 0, @prevCust := 0, @prevDay := 0) prevVals 
ORDER BY c.called, DATE_FORMAT(c.`end`,'%Y-%m-%d') 

但是这个输出为prevCust，prevCompany和prevDate同一行值：

Customer  Company  Date  prevCust prevComp prevDate 

00140443360 08434599117 2014-01-28 00140443360 08434599117 2014-01-28 
00475172558 08434599218 2014-01-27 00475172558 08434599218 2014-01-27 
00475172558 08434599118 2014-01-27 00475172558 08434599118 2014-01-27 

所需的输出应该是：

Customer  Company  Date  prevCust prevComp prevDate 

00140443360 08834599117 2014-01-28  null   null  null 
00475172558 08834599218 2014-01-27 00140443360 08834599117 2014-01-28 
00475136333 08834098771 2014-01-22 00475172558 08834599218 2014-01-27 

Answer 1

这很棘手，因为您不能指定变量和将一个语句中的前一个值拉回来。所以，诀窍是使用另一个变量来保存值。

以下逻辑使用case语句和可怕的= NULL得到前一天。我使用这个，因为我知道它永远不会成立，所以case陈述简单地作为排序变量评估的方法。

你的样品输出不gain，所以我省略了它：

SELECT c.called as customer, c.calling as company, 
     DATE_FORMAT(c.`end`,'%Y-%m-%d') as _date, 
     (case when (@t1 := @prevComp) = NULL then @prevComp  -- never happens 
      when (@prevComp := c.called) = NULL then @prevComp -- never happens 
      else @t1 
     end) as prevComp, 
     (case when (@t2 := @prevCust) = NULL then @prevCust  -- never happens 
      when (@prevCust := c.calling) = NULL then @prevCust -- never happens 
      else @t2 
     end) as prevCust, 
     (case when (@t3 := @prevDate) = NULL then @prevDate  -- never happens 
      when (@prevCust := c.`end`) = NULL then @prevDate -- never happens 
      else DATE_FORMAT(@t3,'%Y-%m-%d') 
     end) as prevCust 
FROM cdrdata_archive c CROSS JOIN 
    (SELECT @prevComp := NULL, @prevCust := NULL, @prevDay := NULL) prevVals 
ORDER BY c.called, DATE_FORMAT(c.`end`,'%Y-%m-%d');