SQL Server 2012: Conditionally Incrementing a counter user ROW_NUMBER() - sql

Question

I am trying to apply ROW_NUMBER() to increment a counter based on particular conditions.
My data looks like this, with the target counter being the Prep column
id DSR PrepIndicator Prep
--------------------------------------
1662835 -1 1 1
1662835 14 2 2
1662835 14 2 3
1662835 20 2 4
1667321 -1 1 1
1667321 30 2 2
1667321 14 2 3
1680648 -1 1 1
1680648 14 2 2
1680648 60 1 1
1680648 14 2 2
1680648 14 2 3
1683870 -1 1 1
1683870 12 2 2
1683870 10 2 3
1683870 60 1 1
1683870 7 2 2
Ignoring the PrepIndicator column for the moment, the business logic I am trying to implement is as follows:
For each of the Id's, starting from 1, increment the Prep counter if the DSR is less than 42.
If it is 42 or greater, reset the Prep counter to 1.
The PrepIndicator, in effect, creates a flag to implement this, in that if PrepIndicator = 1 then Prep = 1. If PrepIndicator = 2, then increment Prep.
I'd prefer to achieve this without the PrepIndicator column if possible.
How would I achieve this conditional increment with ROW_NUMBER()?
I've tried
ROW_NUMBER() OVER (PARTITION BY id, PrepIndicator ORDER BY id)
but it doesn't seem to work when the DSR is >= 42.
Any suggestions or help would be great. Thanks!

Answer 1

First, you will need explicit ordering. "Incrementing the counter" only has meaning if you have a previous value. You can add an IDENTITY column to the table, or use ROW_NUMBER() OVER ORDER BY(/* your logic here */). In your table, you do not even have unique values for the first three columns (see 1680648, 14, 2), so I would think adding an ID is the way to go.
To do what you want to achieve, I believe you must do this in a loop. If you use ROW_NUMBER() you may wish to select into a temporary table. By the nature of your question, the term counter indicates you will have a variable.
UPDATE TableA SET rowId = ROW_NUMBER() OVER(ORDER BY id, DSR, PrepIndicator)
then "conditional" seems to signal a good use of CASE
DECLARE #counter INT = 1
DECLARE #row INT = 1
DECLARE #DSR INT
UPDATE TableA SET Prep = #counter
SET #row = (SELECT rowId FROM TableA WHERE rowId > #row)
WHILE EXISTS( SELECT TOP 1 1 FROM TableA WHERE rowId = #row )
BEGIN
SELECT #DSR = DSR FROM TableA WHERE rowId = #row
SET #counter = CASE WHEN #DSR < 42 THEN #counter + 1 ELSE 1 END
UPDATE TableA SET Prep = #counter WHERE rowId = #row
SET #row = (SELECT rowId FROM TableA WHERE rowId > #row)
END

Answer 2

First, you need to add a primary key because there is no physical order in a SQL table; we can call it IdK. The following code should then give you what you want:
select *, row_number() over (partition by Id, (Select Count (*) from MyTable t2 where t2.idk <= t1.idk and t2.id = t1.id and DSR >= 42) order by idk) prep
from MyTable t1
order by idk
As to why your code doesn't work, this is because the rows are first grouped before the partition/numbering is done. In the case with the two columns id and PrepIndicator for the partition, we get the following intermediary result for the last 5 row before the numbering:
id DSR PrepIndicator Row_Number (Id, PrepIndicator)
1683870 -1 1 1
1683870 60 1 2
1683870 12 2 1
1683870 10 2 2
1683870 7 2 3
Notice that the line with DSR = 60 is now in the second position. This is clearly what you don't want to have. In the case with the Select count(*)..., we have the following result for the last 5 rows after the grouping is done, just before the numbering:
id DSR ...Count() Row_Number (Id, ...Count())
1683870 -1 0 1
1683870 12 0 2
1683870 10 0 3
1683870 60 1 1
1683870 7 1 2
You can notice that in this case, there is no change of position for any row.