sql compute difference between 2 rows

Question

I'm looking for a methodology to compare the difference between 2 rows in the same table. From what I found here (How to get difference between two rows for a column field?) it's almost what I wanted. I have done the following code:

create table #tmpTest
(
    id_fund int null,
    id_ShareType int null,
    ValueDate datetime null,
    VarNAV float null,
    FundPerf float null,
)

insert into #tmpTest(id_fund, id_ShareType, ValueDate, VarNAV)
values(1,1,'20140101',100)
insert into #tmpTest(id_fund, id_ShareType, ValueDate, VarNAV) 
values(1,1,'20140102',20)

update #tmpTest
set hrc.FundPerf = (isnull(hrn.VarNAV, 0) - hrc.VarNAV)/hrc.VarNAV
from #tmpTest hrc 
left join #tmpTest hrn on hrn.ValueDate = (select min(ValueDate) from #tmpTest where ValueDate > hrc.ValueDate)
and hrc.id_fund = hrn.id_fund and hrc.id_ShareType = hrn.id_ShareType 

My issue is that the result I'm computing starts on line 1 instead of line 2.

Hereunder the result I'm obtaining:

id_fund id_ShareType ValueDate           VarNAV                       FundPerf                     
------- ------------ ------------------- ------- -----------------------------
      1            1 2014-01-01 00:00:00     100                          -0.8
      1            1 2014-01-02 00:00:00      20                            -1

whereas I'd like it to be that way:

id_fund id_ShareType ValueDate           VarNAV                       FundPerf                     
------- ------------ ------------------- ------- -----------------------------
      1            1 2014-01-01 00:00:00     100                            -1
      1            1 2014-01-02 00:00:00      20                          -0.8

What's wrong with my approach?

Answer 1

You are not restricting the minimum to the same fund and share type.

update #tmpTest
    set hrc.FundPerf = (isnull(hrn.VarNAV, 0) - hrc.VarNAV)/hrc.VarNAV
    from #tmpTest hrc left join
         #tmpTest hrn
         on hrn.ValueDate = (select min(ValueDate)
                             from #tmpTest tt
                             where tt.ValueDate > hrc.ValueDate and
                                   hrc.id_fund = tt.id_fund and hrc.id_ShareType = tt.id_ShareType 
                            ) and
           hrc.id_fund = hrn.id_fund and hrc.id_ShareType = hrn.id_ShareType ;