Calculating the Weighted Average Cost of products stock

Question

I have to calculate my products stock cost, so for every product after each buy, i have to recalculate the Weighted Average Cost.

I got a view thats bring me the current product's stock after each in/out:

document_type   document_date   product_id  qty_out qty_in  price       row_num stock_balance
SI              01/01/2014      52          0       600     1037.28     1           600
SI              01/01/2014      53          0       300     1357.38     2           300
LC              03/02/2014      53          100     0       1354.16     3           200
LC              03/02/2014      53          150     0       1355.25     4           50
LC              03/02/2014      52          100     0       1035.26     5           500
LC              03/02/2014      52          200     0       1035.04     6           300
LF              03/02/2014      53          0       1040    1356.44     7           1090
LF              03/02/2014      52          0       1560    1045        8           1860
LC              04/02/2014      52          120     0       1039.08     9           1740
LC              04/02/2014      53          100     0       1358.95     10          990
LF              04/02/2014      52          0       600     1038.71     11          2340
LF              04/02/2014      53          0       1040    1363.3      12          2030
LC              05/02/2014      52          100     0       1037.78     13          2240
LF              15/03/2014      53          0       20      1365.87     14          2050
LF              15/03/2014      52          0       50      1054.19     15          2290

I want to add a calculated WAC field as above:

document_type   document_date   product_id  qty_out qty_in  price           row_num     stock_balance   WAC 
SI              01/01/2014      52          0       600     1 037,28        1           600             1037,28000000000
SI              01/01/2014      53          0       300     1 357,38        2           300             1357,38000000000
LC              03/02/2014      53          100     0       1 354,16        3           200             1357,38000000000
LC              03/02/2014      53          150     0       1 355,25        4           50              1357,38000000000
LC              03/02/2014      52          100     0       1 035,26        5           500             1037,28000000000
LC              03/02/2014      52          200     0       1 035,04        6           300             1037,28000000000
LF              03/02/2014      53          0       1040    1 356,44        7           1090            1356,48311926606 --((1357,38*50)+(1040*1356,44))/(1090)
LF              03/02/2014      52          0       1560    1 045,00        8           1860            1043,75483870968 --((1037,28*300)+(1560*1045))/(1860)
LC              04/02/2014      52          120     0       1 039,08        9           1740            1043,75483870968
LC              04/02/2014      53          100     0       1 358,95        10          990             1356,48311926606
LF              04/02/2014      52          0       600     1 038,71        11          2340            1042,46129032258 --((1043,75483870968*1740)+(600*1038,71))/(2340)
LF              04/02/2014      53          0       1040    1 363,30        12          2030            1359,97000000000 --((1356,48311926606*990)+(1040*1363,3))/(2030)
LC              05/02/2014      52          100     0       1 037,78        13          2240            1042,46129032258
LF              15/03/2014      53          0       20      1 365,87        14          2050            1360,03301857239 --((1359,97551136621*2030)+(20*1365,87))/2050
LF              15/03/2014      52          0       50      1 054,19        15          2290            1042.71737568672 --((1042.46129032258*2240)+(50*1054.19))/2290

There is only one and just one document type 'SI' (initial stock) for each product, and the price associated with it is the initial WAC.

Here is a SQL Fiddle sample.

If someone can help with this, i can't figure it out.

Edit: I've juste updated the calculated numbers by increasing precision by displaying 9 numbers after the decimal point.

Answer 1

You need to use recursive CTE:

SQLFiddle

with recursive
stock_temp as (
  select 
    *, 
    row_number() over(partition by product_id order by row_num) as rn
  from 
    stock_table 
)

,cte as (
  select 
    document_type, document_date, 
    product_id, qty_out, qty_in, price, 
    row_num, stock_balance, rn, 
    price as wac
  from 
    stock_temp where document_type = 'SI'

  union all

  select 
    sub.document_type, sub.document_date,
    sub.product_id, sub.qty_out,  sub.qty_in, sub.price,
    sub.row_num, sub.stock_balance,  sub.rn,
    case when sub.qty_in = 0 then main.wac else 
    ((sub.stock_balance - sub.qty_in) * main.wac + sub.qty_in * sub.price) 
      / ((sub.stock_balance - sub.qty_in)  + sub.qty_in) end as wac
  from 
    cte as main
    join stock_temp as sub 
      on (main.product_id = sub.product_id and main.rn + 1 = sub.rn)
)

select * from cte