MySQL：取得上一行

Question

I have the following query:

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')

However this outputs the same row values for prevCust , prevCompany and prevDate :

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

The desired output should be:

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

This is tricky, because you cannot assign the variable and pull the previous value in one statement. So, the trick is to use another variable to hold the value.

The following logic gets the previous day using case statements and the dreaded = NULL . I use this because I know it will never be true, so the case statement simple acts as a method of ordering the evaluation of the variables.

Your sample output doesn't have gain , so I've omitted it:

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');