SQL Server Progressive/Compound Subtraction?

Question

I have a table that looks like the below. I am trying to figure out how to update the "endval" column with a value that is from the "startval" column, reduced by 10% for every multiple of 5 in the "effect" column.

declare @tbl table ( rowid int , effect Int , startval decimal(6,2) , endval decimal(6,2) )
insert @tbl values 
  ( 0 , 10 , 6 , null )   -- expect 4.86 in endval
, ( 1 , 40 , 10 , null  ) -- expect 4.30 in endval
, ( 2 , 7  , 1 , null ) -- expect .9 in endval
select * from @tbl

Note that rowid 2 does not have an even multiple of 5 in the "effect" column, so it is only reduced by 10% once.

I am trying to come up with any way to "progressively percentage" in TSQL (2012) and nothing is coming to mind. Help?

Thanks.

Answer 1

Use POWER to apply the multiple percentages.

declare @tbl table ( rowid int , effect Int , startval decimal(6,2) , endval decimal(6,2) )
insert @tbl values 
      ( 0 , 10 , 6 , null )   -- expect 4.86 in endval
    , ( 1 , 40 , 10 , null  ) -- expect 4.30 in endval
    , ( 2 , 7  , 1 , null ) -- expect .9 in endval

select  rowid, startval, [endval]=power(0.90, effect/5)*startval
from    @tbl;

Results:

rowid   startval    endval
0       6.00        4.8600
1       10.00       4.3000
2       1.00        0.9000

A simple loop in a cte will also get it done:

;with cte (rowid, calc, i) as
(
    select  rowid, startval, effect/5
    from    @tbl
    union all
    select  t.rowid, cast(calc*.9 as decimal(6,2)), i-1
    from    @tbl t
    join    cte c on 
            c.rowid = t.rowid
    where   c.i > 0
)
select  * 
from    cte c
where   i = 0
order
by      rowid;

Results:

rowid   calc    i

0       4.86    0
1       4.30    0
2       0.90    0