How to update a column based on values of other columns

I have a tables as below

row_wid id code sub_code item_nbr orc_cnt part_cnt variance reporting_date var_start_date
1      1   ABC   PQR     23AB      0       1        1   11-10-2019  NULL
2      1   ABC   PQR     23AB      0       1        1   12-10-2019  NULL
3      1   ABC   PQR     23AB      1       1        0   13-10-2019  NULL
4      1   ABC   PQR     23AB      1       2        1   14-10-2019  NULL
5      1   ABC   PQR     23AB      1       3        2   15-10-2019  NULL

I have to update var_start_date column with min(reporting_date) for each combination of id,code,sub_code and item_nbr only till variance field is zero. Row with variance = 0 should have null var_start_date. and next row after that should have next min(var_start_date.). FYI, variance is calculated as par_cnt-orc_cnt

so my output should look like this –

row_wid id code sub_code item_nbr orc_cnt part_cnt variance reporting_date var_start_date
1      1   ABC   PQR     23AB      0       1        1   11-10-2019  11-10-2019
2      1   ABC   PQR     23AB      0       1        1   12-10-2019  11-10-2019
3      1   ABC   PQR     23AB      1       1        0   13-10-2019  NULL
4      1   ABC   PQR     23AB      1       2        1   14-10-2019  14-10-2019
5      1   ABC   PQR     23AB      1       3        2   15-10-2019  14-10-2019

I am trying to write a function using below query to divide the data into sets.

SELECT DISTINCT MIN(reporting_date) 
        OVER (partition by id, code,sub_code,item_nbr ORDER BY row_wid ),
        RANK() OVER (partition by id, code,sub_code,item_nbr ORDER BY row_wid)
        AS rnk,id, code,sub_code,item_nbr,orc_cnt,part_cnt,variance,row_wid
FROM TABLE T1

.But dont know how to include variance field to split the sets.

Answer

Try as below

SELECT T.*, CASE WHEN T.variance = 0 THEN NULL ELSE MIN(reporting_date) OVER (PARTITION BY T1.RANK ORDER BY T1.RANK) END AS New_var_start_date 
FROM mytbl T
LEFT JOIN (
           SELECT row_wid, variance, COUNT(CASE variance WHEN 0 THEN 1 END) OVER (ORDER BY row_wid ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) +1 AS [Rank]
           FROM mytbl 
          ) T1 ON T.row_wid = T1.row_wid

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