delete certain amount of rows with group by is possible? - sql

Question

So I have in one table called "Park" and another table called "WaterSource". The idea is in each line of Park say how much "waterSources" I have and create that amount of lines in the second table. When I change the value of Park to a minor number (like I had 5 and now I want to have 3) I need to delete the "last" rows of "water source" for the last 2 "Parks".
I need to do something like this,
BEGIN
declare #x int;
set #x = 1;
while #x > 0
BEGIN
delete top (#diff) from [WaterSource]
where [IdView] = #IdView
order by [Park] DESC;
set #x = ##ROWCOUNT;
END
END
I don't know how to do this because the only parameter in these second table is IdView and I just want to "group" all lines for "Park" order by Park Id desc and delete all.
http://www.sqlfiddle.com/#!9/2cbe3fb/1
So my idea is if I change parks number to 1 the lines with idPark 2 should all be deleted and Park 2 will be deleted also. I always delete by the last to the first.

Answer 1

Based on the sample provided I assume that SQL Server is the database engine. The key to my solution is creating a temporary table with a rank or row number column that is created by the windowed function ROW_NUMBER() (More info on windowed functions here: http://www.sqlservertutorial.net/sql-server-window-functions/)
DROP TABLE IF EXISTS #ranker
DECLARE #ParkId INT = 2
DECLARE #newNumWaterSources INT = (SELECT numWaterSources FROM Parks WHERE id = #ParkId)
SELECT
ws.id,
ROW_NUMBER() OVER(ORDER BY ws.id) AS rn
INTO #ranker
FROM WaterSources ws
WHERE idPark = #ParkId
DELETE FROM WaterSources
WHERE idPark = #ParkId
AND id IN
(SELECT id FROM #ranker WHERE rn > #newNumWaterSources)
There are many ways to automate this so that the two variables are populated with the most relevant values. You may want to play around with using an AFTER UPDATE TRIGGER to query the deleted table for the park table id.
This solution would only work if the id column in the WaterSources table is unique. So you would need to change the insert to WaterSources in your fiddle to the following:
INSERT INTO WaterSources VALUES (1, 1, 37);
INSERT INTO WaterSources VALUES (2, 1, 37);
INSERT INTO WaterSources VALUES (3, 1, 37);
INSERT INTO WaterSources VALUES (4, 2, 37);
INSERT INTO WaterSources VALUES (5, 2, 37);
INSERT INTO WaterSources VALUES (6, 2, 37);