create a row index column beginning at -1 - sql

Question

I need to create a row index column that begins at -1 so i can query the previous day's balance. My current query:
select TRANSDATE, sum(convert(float,AMOUNTMST-SETTLEAMOUNTMST)) as Balance
from [AX2cTestStage].[dbo].[CUSTTRANS_V]
group by TRANSDATE
order by TRANSDATE asc
TRANSDATE Balance
2019-04-12 00:00:00.000 -22591.47
2019-04-15 00:00:00.000 -394.95
2019-04-25 00:00:00.000 -1776
2019-04-26 00:00:00.000 -11973.84
2019-04-29 00:00:00.000 -24230.16
2019-05-02 00:00:00.000 -10695.39
This is what i need:
TRANSDATE Balance Row Index
2019-04-12 00:00:00.000 -22591.47 -1
2019-04-15 00:00:00.000 -394.95 0
2019-04-25 00:00:00.000 -1776 1
2019-04-26 00:00:00.000 -11973.84 2
2019-04-29 00:00:00.000 -24230.16 3
2019-05-02 00:00:00.000 -10695.39 4
I have tried to declare a variable as the row index
declare #row_num as int = -1
select TRANSDATE, sum(convert(float,AMOUNTMST-SETTLEAMOUNTMST)) as Balance, #row_num += 1 as Row Index
from [AX2cTestStage].[dbo].[CUSTTRANS_V]
group by TRANSDATE
i receive this error:
A SELECT statement that assigns a value to a variable must not be combined with data-retrieval operations.
after declaring a variable for each field I still receives errors. Is there an easier way to accomplish this? thanks

Answer 1

You can use ROW_NUMBER(). For example:
select
TRANSDATE,
sum(convert(float,AMOUNTMST-SETTLEAMOUNTMST)) as Balance,
row_number() over(order by TRANSDATE) - 2 as Row Index
from [AX2cTestStage].[dbo].[CUSTTRANS_V]
group by TRANSDATE