SQL Server - loop through table and update based on count - sql

Question

I have a SQL Server database. I need to loop through a table to get the count of each value in the column 'RevID'. Each value should only be in the table a certain number of times - for example 125 times. If the count of the value is greater than 125 or less than 125, I need to update the column to ensure all values in the RevID (are over 25 different values) is within the same range of 125 (ok to be a few numbers off)
For example, the count of RevID = "A2" is = 45 and the count of RevID = 'B2' is = 165 then I need to update RevID so the 45 count increases and the 165 decreases until they are within the 125 range.
This is what I have so far:
DECLARE #i INT = 1,
#RevCnt INT = SELECT RevId, COUNT(RevId) FROM MyTable group by RevId
WHILE(#RevCnt >= 50)
BEGIN
UPDATE MyTable
SET RevID= (SELECT COUNT(RevID) FROM MyTable)
WHERE RevID < 50)
#i = #i + 1
END
I have also played around with a cursor and instead of trigger. Any idea on how to achieve this? Thanks for any input.

Answer 1

Okay I cam back to this because I found it interesting even though clearly there are some business rules/discussion that you and I and others are not seeing. anyway, if you want to evenly and distribute arbitrarily there are a few ways you could do it by building recursive Common Table Expressions [CTE] or by building temp tables and more. Anyway here is a way that I decided to give it a try, I did utilize 1 temp table because sql was throwing in a little inconsistency with the main logic table as a cte about every 10th time but the temp table seems to have cleared that up. Anyway, this will evenly spread RevId arbitrarily and randomly assigning any remainder (# of Records / # of RevIds) to one of the RevIds. This script also doesn't rely on having a UniqueID or anything it works dynamically over row numbers it creates..... here you go just subtract out test data etc and you have what you more than likely want. Though rebuilding the table/values would probably be easier.
--Build Some Test Data
DECLARE #Table AS TABLE (RevId VARCHAR(10))
DECLARE #C AS INT = 1
WHILE #C <= 400
BEGIN
IF #C <= 200
BEGIN
INSERT INTO #Table (RevId) VALUES ('A1')
END
IF #c <= 170
BEGIN
INSERT INTO #Table (RevId) VALUES ('B2')
END
IF #c <= 100
BEGIN
INSERT INTO #Table (RevId) VALUES ('C3')
END
IF #c <= 400
BEGIN
INSERT INTO #Table (RevId) VALUES ('D4')
END
IF #c <= 1
BEGIN
INSERT INTO #Table (RevId) VALUES ('E5')
END
SET #C = #C+ 1
END
--save starting counts of test data to temp table to compare with later
IF OBJECT_ID('tempdb..#StartingCounts') IS NOT NULL
BEGIN
DROP TABLE #StartingCounts
END
SELECT
RevId
,COUNT(*) as Occurences
INTO #StartingCounts
FROM
#Table
GROUP BY
RevId
ORDER BY
RevId
/************************ This is the main method **********************************/
--clear temp table that is the main processing logic
IF OBJECT_ID('tempdb..#RowNumsToChange') IS NOT NULL
BEGIN
DROP TABLE #RowNumsToChange
END
--figure out how many records there are and how many there should be for each RevId
;WITH cteTargetNumbers AS (
SELECT
RevId
--,COUNT(*) as RevIdCount
--,SUM(COUNT(*)) OVER (PARTITION BY 1) / COUNT(*) OVER (PARTITION BY 1) +
--CASE
--WHEN ROW_NUMBER() OVER (PARTITION BY 1 ORDER BY NEWID()) <=
--SUM(COUNT(*)) OVER (PARTITION BY 1) % COUNT(*) OVER (PARTITION BY 1)
--THEN 1
--ELSE 0
--END as TargetNumOfRecords
,SUM(COUNT(*)) OVER (PARTITION BY 1) / COUNT(*) OVER (PARTITION BY 1) +
CASE
WHEN ROW_NUMBER() OVER (PARTITION BY 1 ORDER BY NEWID()) <=
SUM(COUNT(*)) OVER (PARTITION BY 1) % COUNT(*) OVER (PARTITION BY 1)
THEN 1
ELSE 0
END - COUNT(*) AS NumRecordsToUpdate
FROM
#Table
GROUP BY
RevId
)
, cteEndRowNumsToChange AS (
SELECT *
,SUM(CASE WHEN NumRecordsToUpdate > 1 THEN NumRecordsToUpdate ELSE 0 END)
OVER (PARTITION BY 1 ORDER BY RevId) AS ChangeEndRowNum
FROM
cteTargetNumbers
)
SELECT
*
,LAG(ChangeEndRowNum,1,0) OVER (PARTITION BY 1 ORDER BY RevId) as ChangeStartRowNum
INTO #RowNumsToChange
FROM
cteEndRowNumsToChange
;WITH cteOriginalTableRowNum AS (
SELECT
RevId
,ROW_NUMBER() OVER (PARTITION BY RevId ORDER BY (SELECT 0)) as RowNumByRevId
FROM
#Table t
)
, cteRecordsAllowedToChange AS (
SELECT
o.RevId
,o.RowNumByRevId
,ROW_NUMBER() OVER (PARTITION BY 1 ORDER BY (SELECT 0)) as ChangeRowNum
FROM
cteOriginalTableRowNum o
INNER JOIN #RowNumsToChange t
ON o.RevId = t.RevId
AND t.NumRecordsToUpdate < 0
AND o.RowNumByRevId <= ABS(t.NumRecordsToUpdate)
)
UPDATE o
SET RevId = u.RevId
FROM
cteOriginalTableRowNum o
INNER JOIN cteRecordsAllowedToChange c
ON o.RevId = c.RevId
AND o.RowNumByRevId = c.RowNumByRevId
INNER JOIN #RowNumsToChange u
ON c.ChangeRowNum > u.ChangeStartRowNum
AND c.ChangeRowNum <= u.ChangeEndRowNum
AND u.NumRecordsToUpdate > 0
IF OBJECT_ID('tempdb..#RowNumsToChange') IS NOT NULL
BEGIN
DROP TABLE #RowNumsToChange
END
/***************************** End of Main Method *******************************/
-- Compare the results and clean up
;WITH ctePostUpdateResults AS (
SELECT
RevId
,COUNT(*) as AfterChangeOccurences
FROM
#Table
GROUP BY
RevId
)
SELECT *
FROM
#StartingCounts s
INNER JOIN ctePostUpdateResults r
ON s.RevId = r.RevId
ORDER BY
s.RevId
IF OBJECT_ID('tempdb..#StartingCounts') IS NOT NULL
BEGIN
DROP TABLE #StartingCounts
END

Answer 2

Since you've given no rules for how you'd like the balance to operate we're left to speculate. Here's an approach that would find the most overrepresented value and then find an underrepresented value that can take on the entire overage.
I have no idea how optimal this is and it will probably run in an infinite loop without more logic.
declare #balance int = 125;
declare #cnt_over int;
declare #cnt_under int;
declare #revID_overrepresented varchar(32);
declare #revID_underrepresented varchar(32);
declare #rowcount int = 1;
while #rowcount > 0
begin
select top 1 #revID_overrepresented = RevID, #cnt_over = count(*)
from T
group by RevID
having count(*) > #balance
order by count(*) desc
select top 1 #revID_underrepresented = RevID, #cnt_under = count(*)
from T
group by RevID
having count(*) < #balance - #cnt_over
order by count(*) desc
update top #cnt_over - #balance T
set RevId = #revID_underrepresented
where RevId = #revID_overrepresented;
set #rowcount = ##rowcount;
end

Answer 3

The problem is I don't even know what you mean by balance...You say it needs to be evenly represented but it seems like you want it to be 125. 125 is not "even", it is just 125.
I can't tell what you are trying to do, but I'm guessing this is not really an SQL problem. But you can use SQL to help. Here is some helpful SQL for you. You can use this in your language of choice to solve the problem.
Find the rev values and their counts:
SELECT RevID, COUNT(*)
FROM MyTable
GROUP BY MyTable
Update #X rows (with RevID of value #RevID) to a new value #NewValue
UPDATE TOP #X FROM MyTable
SET RevID = #NewValue
WHERE RevID = #RevID
Using these two queries you should be able to apply your business rules (which you never specified) in a loop or whatever to change the data.