Assign values to variables in stored procedure - sql

Question

select #numOfColumns = count(distinct Col) from #b
SET #sql2=
'SELECT'+ #columns +'+= QUOTENAME(Col) + '',''
from (SELECT DISTINCT top #numOfColumns Col FROM #b ORDER BY Col) A';
EXECUTE sp_executesql #sql2;
I am trying to get this stored procedure to work. Trying to pass #numOfColumns to the statement then assign the values from QUOTENAME(Col) to #columns and Then Exec the statement.
Script
DECLARE
#columns NVARCHAR(MAX) = '',
#sql NVARCHAR(MAX) = '',
#sql2 NVARCHAR(MAX) = '',
#numOfColumns int = 0;
SELECT customerid, curbal, Col = CAST (ROW_NUMBER() OVER (PARTITION BY
convert(int,customerid) ORDER BY convert(float,curbal) desc) as int)
into #b
FROM [sav acc]
select #numOfColumns = count(distinct Col) from #b
SET #sql2= 'SELECT '+ #columns+' += QUOTENAME(Col) + '','' from (SELECT
DISTINCT top (#numOfColumns) Col FROM #b ORDER BY Col) A';
EXECUTE sp_executesql #sql2, N'#numOfColumns', #numOfColumns int;
This is origin of post. ORIGINAL POST . The solution did work, But i tested it with the Source data it would positions column at [3], [2], 1 and and I wanted the columns as 1,[2],[3]

Answer 1

Firstly, the way you are using TOP is pointless. If you have 5 distinct values for Col in #b then SELECT DISTINCT Col FROM #b will only return 5 records, regardless of if you apply TOP 5 or not. It seems you are only using it to allow sorting, but you could just put the sort in the outer query.
Secondly, you should not use variable assignment to concatenate strings, the behaviour is unreliable and undocumented you should use STRING_AGG, or for earlier versions of SQL Server you can leverage the XML extensions, either:
CREATE TABLE #B (Col CHAR(1));
INSERT #B (Col) VALUES ('A'), ('A'), ('B'), ('C');
DECLARE #Columns NVARCHAR(MAX) =
( SELECT STRING_AGG(QUOTENAME(Col), ',') WITHIN GROUP (ORDER BY Col)
FROM (SELECT DISTINCT Col FROM #B) AS b
);
Or
DECLARE #Columns NVARCHAR(MAX) =
STUFF(( SELECT CONCAT(',', QUOTENAME(Col))
FROM #B
GROUP BY Col
ORDER BY Col
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)'), 1, 1, '');
While the above shows that dynamic sql is unnecessary, it would also be unnecessary if you were using #top, you could simply use:
DECLARE #numOfColumns INT = (SELECT count(distinct Col) from #b);
DECLARE #Columns NVARCHAR(MAX) = '';
SELECT #Columns += QUOTENAME(Col) + ','
FROM (SELECT DISTINCT TOP (#numOfColumns) Col FROM #B) AS b
ORDER BY Col;
SELECT #Columns;