Retrieve Sorted Column Value in SQL Server - sql-server

Question

What i have:
I have a Column
ID  SerialNo
1  101
2  102
3  103
4  104
5  105
6  116
7  117
8  118
9  119
10 120
These are just the 10 dummy rows. The actual table has over 100 000 rows.
What I Want to get:
A method or formula like any sorting technique which could return me the starting and ending element of [SerialNo] Column for every sub-series. For example
Expected Result: 101-105, 115-120
The comma separation in the above result is not important, only the starting and ending elements are important.
What I have tried:
I did it by PL/SQL programming, by running a loop in which I’m getting the starting and ending elements getting stored in a TABLE.
But due to no. of rows (over 100 000) the query execution is taking around 2 minutes.
I have also searched about some sorting techniques for the SQL Server but I found nothing. Because rendering every row will take twice the time then a sorting algorithm

Answer 1

Assuming every sub series should contain 5 records, I got expected result using below sql. I hope this helps.
DECLARE #subSeriesRange INT=5;
CREATE TABLE #Temp(ID INT,SerialNo INT);
INSERT INTO #Temp VALUES(1,101),
(2,102),
(3,103),
(4,104),
(5,105),
(6,116),
(7,117),
(8,115),
(9,119),
(10,120);
SELECT STUFF((SELECT CONCAT(CASE ID%#subSeriesRange WHEN 1 THEN ',' ELSE '-' END,SerialNo)
FROM #Temp
WHERE ID%#subSeriesRange = 1 OR ID%#subSeriesRange=0
ORDER BY ID
FOR XML PATH('')),1,1,''
);
DROP TABLE #Temp;

Answer 2

Just finding the start and end of each series is quite straightforward:
declare #t table (ID int not null, SerialNo int not null)
insert into #t(ID,SerialNo) values
(1 ,101), (2 ,102), (3 ,103),
(4 ,104), (5 ,105), (6 ,116),
(7 ,117), (8 ,118), (9 ,119),
(10,120)
;With Starts as (
select t1.SerialNo,ROW_NUMBER() OVER (ORDER BY t1.SerialNo) as rn
from
#t t1
left join
#t t1_no
on t1.SerialNo = t1_no.SerialNo + 1
where t1_no.ID is null
), Ends as (
select t1.SerialNo,ROW_NUMBER() OVER (ORDER BY t1.SerialNo) as rn
from
#t t1
left join
#t t1_no
on t1.SerialNo = t1_no.SerialNo - 1
where t1_no.ID is null
)
select
s.SerialNo as StartSerial,
e.SerialNo as EndSerial
from
Starts s
inner join
Ends e
on s.rn = e.rn
The logic being that a Start is a row where there is no row that has the SerialNo one less than the current row, and an End is a row where there is no row that has the SerialNo one more than the current row.
This may still perform poorly if there is no index on the SerialNo column.
Results:
StartSerial EndSerial
----------- -----------
101 105
116 120
Which is hopefully acceptable since you didn't seem to care what the specific results look like. It's also keeping things set-based.