How to write this excel formulae below in sql see problem below with example

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SQL

A        B     C           D            E
ItemID  weight Service     Eventdate   weightgain or lost(Col B)
16568906    24  Programme   05/10/2015  
16568906    24  Trainer     05/10/2015      0
16568906    29  Programme   28/01/2016      5
16568906    29  Trainer     28/01/2016      0
168906      23  Trainer     29/01/2016  
168906      20  Trainer     30/01/2016     -3
1656890610  28  Programme   05/10/2015  
1656890610  35  Programme   06/10/2015      7
1656890610  30  Programme   07/10/2015     -5

=IF(A1=A2,B2-B1) give result in col E

What I have tried:

I'm trying to replicate an excel formulae in Sql and I have try to use cumulative value but never work