T-SQL: Efficient way to add up column values - sql

Question

Now I'm sure this has been asked and superbly been answered on here. However, I am unable to find the answer since it touches many keywords.
I basically want to replace a table of the form:
Type amount param note
7 2 str1 NULL
42 12 str2 NULL
128 7 str3 samplenote
42 12 NULL NULL
101 4 str4 NULL
42 12 NULL NULL
7 1 str1 samplenote
128 2 str5 NULL
with a table like:
Type amount param note
7 3 str1 combined
42 36 NULL combined
128 9 NULL combined
101 4 str4 combined
In words, I seek to sum up the amount parameter based on its type while declaring param = NULL for all "unclear" fields. (param should be NULL when the param values of combined Types have more than one different content; else, param should have the original content.)
With my python background, I tackled this task with a for loop approach, iterating through the types, adding a new row for every type with summed up amount and note = 'combined', to then delete the remaining rows (see below). There has to be a more efficient way with some JOIN statement I'm sure. But how would that look like?
FYI, this is the solution I am working on (not functioning yet!):
/*** dbo.sourcetable holds all possible Type values ***/
CREATE PROCEDURE [sumup]
AS
BEGIN
DECLARE #i int = (SELECT TOP (1) Type FROM [dbo].[sourcetable] ORDER BY Type)
DECLARE #MaxType int = (SELECT TOP (1) Type FROM [dbo].[sourcetable] ORDER BY Type DESC)
DECLARE #sum int
BEGIN TRY
WHILE #i <= #MaxType
BEGIN
IF EXISTS (SELECT * FROM [dbo].[worktable] WHERE Type = #i)
BEGIN
SET #sum = (SELECT SUM(amount) FROM [dbo].[worktable] WHERE Type = #i)
BEGIN
WITH cte AS (SELECT * FROM [dbo].[worktable] WHERE Type = #i)
INSERT INTO [dbo].[worktable]
([Type]
,[amount]
,[param]
,[note]
SELECT
cte.Type
,#sum
,cte.param
,'combined'
FROM cte
END
DELETE FROM [dbo].[worktable] WHERE Type = #i AND ISNULL([note],'') <> 'combined'
END
SET #i = #i + 1
END
END TRY
BEGIN CATCH
-- some errorlogging code
END CATCH
END
GO

Answer 1

This can be achieved with a single select statement.
If you require your combined flag to only apply to where more than one row has been combined, add another case expression checking the result of either a count(1) for rows combined or count(distinct param) for unique param values combined:
declare #t as table(type int, amount int, param varchar(15), note varchar(15));
insert into #t values (7,2,'str1',NULL),(42,12,'str2',NULL),(128,7,'str3','samplenote'),(42,12,NULL,NULL),(101,4,'str4',NULL),(42,12,NULL,NULL),(7,1,'str1','samplenote'),(128,2,'str5',NULL);
select type
,sum(amount) as amount
,case when count(distinct isnull(param,'')) = 1
then max(param)
else null
end as param
,'combined' as note
from #t
group by type
order by type;
Output:
+------+--------+-------+----------+
| type | amount | param | note |
+------+--------+-------+----------+
| 7 | 3 | str1 | combined |
| 42 | 36 | NULL | combined |
| 101 | 4 | str4 | combined |
| 128 | 9 | NULL | combined |
+------+--------+-------+----------+

Answer 2

I am doing this way from keyboard, but this may work or be close to what you want
Select type , amount , iif( dc=1,p,null) param, 'combined' note
from
(
Select type, sum(amount) amount,
count(distinct Param) dc,max(Param) p
From ....
Group by type
) x

Answer 3

Here is a possible solution:
declare #tbl as table (
type int
,amount int
,param varchar(15)
,note varchar(15)
)
insert into #tbl values (7,2,'str1',NULL)
insert into #tbl values (42,12,'str2',NULL)
insert into #tbl values (128,7,'str3','samplenote')
insert into #tbl values (42,12,NULL,NULL)
insert into #tbl values (101,4,'str4',NULL)
insert into #tbl values (42,12,NULL,NULL)
insert into #tbl values (7,1,'str1','samplenote')
insert into #tbl values (128,2,'str5',NULL)
;WITH CTE AS (
SELECT
type
,SUM(AMOUNT) AS amount
,COUNT(DISTINCT ISNULL(param, 'dummy value')) AS ParamNo
,MAX(Param) AS Param
FROM #tbl
GROUP BY type
) SELECT
type
,amount
,CASE WHEN ParamNo = 1 THEN Param ELSE NULL END AS Param
,'combined' AS note
FROM CTE

Answer 4

This should work:
Select Type, sum(amount) as amount, count(distinct param)
, case when count(distinct param) = 1 then max(param) end as param,
'Combined' as note
From
mytable
Group By Type