Merging data in a single SQL table without a Cursor - sql

Question

I have a table with an ID column and another column with a number. One ID can have multiple numbers. For example
ID | Number
1 | 25
1 | 26
1 | 30
1 | 24
2 | 4
2 | 8
2 | 5
Now based of this data, in a new table, I want to have this
ID | Low | High
1 | 24 | 26
1 | 30 | 30
2 | 4 | 5
2 | 8 | 8
As you can see, I want to merge any data where the numbers are consecutive, like 24, 25, 26. So now the low was 24, the high was 26, and then 30 is still a separate range. I am dealing with large amounts of data, so I would prefer to not use a cursor for performance sake (which is what I was previously doing, and was slowing things down quite a bit)...What is the best way to achieve this? I'm no SQL pro, so I'm not sure if there is a function available that could make this easier, or what the fastest way to accomplish this would be.
Thanks for the help.

Answer 1

The key observation is that a sequence of numbers minus another sequence is a constant. We can generate another sequence using row_number. This identifies all the groups:
select id, MIN(number) as low, MAX(number) as high
from (select t.*,
(number - ROW_NUMBER() over (partition by id order by number) ) as groupnum
from t
) t
group by id, groupnum
The rest is just aggregation.

Answer 2

Solution with CTE and recursion:
WITH CTE AS (
SELECT T.ID, T.NUMBER, T.NUMBER AS GRP
FROM T
LEFT OUTER JOIN T T2 ON T.ID = T2.ID AND T.NUMBER -1 = T2.NUMBER
WHERE T2.ID IS NULL
UNION ALL
SELECT T.ID, T.NUMBER, GRP
FROM CTE
INNER JOIN T
ON T.ID = CTE.ID AND T.NUMBER = CTE.NUMBER + 1
)
SELECT ID, MAX( NUMBER ), MIN(NUMBER)
FROM CTE
GROUP BY ID, GRP
Results at fiddlesql

Answer 3

I'd suggest using a WHILE loop structure with a table variable instead of the cursor.
For example,
DECLARE #TableVariable TABLE
(
MyID int IDENTITY (1, 1) PRIMARY KEY NOT NULL,
[ID] int,
[Number] int
)
DECLARE #Count int, #Max int
INSERT INTO #TableVariable (ID, Number)
SELECT ID, Number
FROM YourSourceTable
SELECT #Count = 1, #Max = MAX(MyID)
FROM #TableVariable
WHILE #Count <= #Max
BEGIN
...do your processing here...
SET #Count = #Count + 1
END

Answer 4

CREATE TABLE Table1
([ID] int, [Number] int)
;
INSERT INTO Table1
([ID], [Number])
VALUES
(1, 25),
(1, 26),
(1, 30),
(1, 24),
(2, 4),
(2, 8),
(2, 5)
;
select ID,
MIN(Number)
,(SELECT MIN(Number)
FROM (SELECT TOP 2 Number from Table1 WHERE ID =
T1.Id ORDER BY Number DESC) as DT)
from Table1 as T1
GROUP BY ID
UNION
SELECT ID, MAX(Number), MAX(Number)
FROM Table1 as T1
GROUP BY ID;
Live Example