Update using Self Join Sql Server - sql

Question

I have huge data and sample of the table looks like below
+-----------+------------+-----------+-----------+
| Unique_ID | Date | RowNumber | Flag_Date |
+-----------+------------+-----------+-----------+
| 1 | 6/3/2014 | 1 | 6/3/2014 |
| 1 | 5/22/2015 | 2 | NULL |
| 1 | 6/3/2015 | 3 | NULL |
| 1 | 11/20/2015 | 4 | NULL |
| 2 | 2/25/2014 | 1 | 2/25/2014 |
| 2 | 7/31/2014 | 2 | NULL |
| 2 | 8/26/2014 | 3 | NULL |
+-----------+------------+-----------+-----------+
Now I need to check if the difference between Date in 2nd row and Flag_date in 1st row. If the difference is more than 180 then 2nd row Flag_date should be updated with the date in 2nd row else it needs to be updated by Flag_date in 1st Row. And same rule follows for all rows with same unique_ID
update a
set a.Flag_Date=case when DATEDIFF(dd,b.Flag_Date,a.[Date])>180 then a.[Date] else b.Flag_Date end
from Table1 a
inner join Table1 b
on a.RowNumber=b.RowNumber+1 and a.Unique_ID=b.Unique_ID
The above update query when executed once, only the second row under each Unique_ID gets updated and result looks like below
+-----------+------------+-----------+------------+
| Unique_ID | Date | RowNumber | Flag_Date |
+-----------+------------+-----------+------------+
| 1 | 2014-06-03 | 1 | 2014-06-03 |
| 1 | 2015-05-22 | 2 | 2015-05-22 |
| 1 | 2015-06-03 | 3 | NULL |
| 1 | 2015-11-20 | 4 | NULL |
| 2 | 2014-02-25 | 1 | 2014-02-25 |
| 2 | 2014-07-31 | 2 | 2014-02-25 |
| 2 | 2014-08-26 | 3 | NULL |
+-----------+------------+-----------+------------+
And I need to run four times to achieve my desired result
+-----------+------------+-----------+------------+
| Unique_ID | Date | RowNumber | Flag_Date |
+-----------+------------+-----------+------------+
| 1 | 2014-06-03 | 1 | 2014-06-03 |
| 1 | 2015-05-22 | 2 | 2015-05-22 |
| 1 | 2015-06-03 | 3 | 2015-05-22 |
| 1 | 2015-11-20 | 4 | 2015-11-20 |
| 2 | 2014-02-25 | 1 | 2014-02-25 |
| 2 | 2014-07-31 | 2 | 2014-02-25 |
| 2 | 2014-08-26 | 3 | 2014-08-26 |
+-----------+------------+-----------+------------+
Is there a way where I can run update only once and all the rows are updated.
Thank you!

Answer 1

If you are using SQL Server 2012+, then you can use lag():
with toupdate as (
select t1.*,
lag(flag_date) over (partition by unique_id order by rownumber) as prev_flag_date
from table1 t1
)
update toupdate
set Flag_Date = (case when DATEDIFF(day, prev_Flag_Date, toupdate.[Date]) > 180
then toupdate.[Date] else prev_Flag_Date
end);
Both this version and your version can take advantage of an index on table1(unique_id, rownumber) or, better yet, table1(unique_id, rownumber, flag_date).
EDIT:
In earlier versions, this might have better performance:
with toupdate as (
select t1.*, t2.flag_date as prev_flag_date
from table1 t1 outer apply
(select top 1 t2.flag_date
from table1 t2
where t2.unique_id = t1.unique_id and
t2.rownumber < t1.rownumber
order by t2.rownumber desc
) t2
)
update toupdate
set Flag_Date = (case when DATEDIFF(day, prev_Flag_Date, toupdate.[Date]) > 180
then toupdate.[Date] else prev_Flag_Date
end);
The CTE can make use of the same index -- and it is important to have the index. The reason for the better performance is because your join on row_number() cannot use an index on that field.