Reorder columns in final stored procedure SELECT / OUTPUT - sql

Question

I need to reorder columns in the final SELECT statement in a stored procedure. Column orders needs to be fetched from another table.
I have a solution based on dynamic SQL. Is there any better way to do it? I have around 100 columns to return with millions of rows for an Excel export. Is there any other performance optimized solution other than a dynamic query?
Please find sample code below for my current solution.
IF OBJECT_ID( 'tempdb..#TempColumns') IS NOT NULL
BEGIN
DROP TABLE #TempColumns
END
IF OBJECT_ID( 'tempdb..#TempColumnsOrder') IS NOT NULL
BEGIN
DROP TABLE #TempColumnsOrder
END
CREATE TABLE #TempColumns
(
ID INT IDENTITY,
FirstName VARCHAR(MAX),
LastName VARCHAR(MAX),
Gender VARCHAR(MAX)
)
INSERT INTO #TempColumns
VALUES ('ABC', 'DEF', 'MALE'), ('PR', 'ZA', 'FEMALE'), ('ERT', 'GFG', 'MALE')
CREATE TABLE #TempColumnsOrder
(
ID INT IDENTITY,
ColumnName VARCHAR(MAX),
ColumnOrder INT
)
INSERT INTO #TempColumnsOrder
VALUES ('FirstName', 3), ('LastName', 2), ('Gender', 1)
SELECT * FROM #TempColumns
SELECT * FROM #TempColumnsOrder
DECLARE #script VARCHAR(MAX)
SELECT #script = 'SELECT '
SELECT #script = #script + ColumnName + ','
FROM #TempColumnsOrder
ORDER BY ColumnOrder
PRINT #script
SELECT #script = SUBSTRING(RTRIM(#script), 1, LEN(RTRIM(#script)) - 1)
SELECT #script = #script + ' FROM #TempColumns'
EXEC (#script)
IF OBJECT_ID( 'tempdb..#TempColumns') IS NOT NULL
BEGIN
DROP TABLE #TempColumns
END
IF OBJECT_ID( 'tempdb..#TempColumnsOrder') IS NOT NULL
BEGIN
DROP TABLE #TempColumnsOrder
END

Answer 1

Thanks for reply, Is there any better way in Dynamic SQL other than what i did?
You can eliminate the unsupported string concatenation you are using, and modernize and simply the code:
DROP TABLE IF EXISTS #TempColumns
DROP TABLE IF EXISTS #TempColumnsOrder
CREATE TABLE #TempColumns
(
ID INT IDENTITY,
FirstName VARCHAR(MAX),
LastName VARCHAR(MAX),
Gender VARCHAR(MAX)
)
INSERT INTO #TempColumns
Values('ABC','DEF','MALE'),('PR','ZA','FEMALE'),('ERT','GFG','MALE')
CREATE TABLE #TempColumnsOrder
(
ID INT IDENTITY,
ColumnName VARCHAR(MAX),
ColumnOrder INT
)
INSERT INTO #TempColumnsOrder
Values('FirstName',3), ('LastName',2), ('Gender',1)
SELECT * FROM #TempColumns
SELECT * FROM #TempColumnsOrder
DECLARE #script VARCHAR(MAX) = concat(
'SELECT ',
(select STRING_AGG(QUOTENAME(ColumnName),', ') WITHIN GROUP (ORDER BY ColumnOrder)
FROM #TempColumnsOrder),
' FROM #TempColumns')
print #script
EXEC (#script)
DROP TABLE IF EXISTS #TempColumns
DROP TABLE IF EXISTS #TempColumnsOrder

Answer 2

SELECT #script = #script + ColumnName + ',' FROM #TempColumnsOrder
ORDER BY ColumnOrder
The behavior of aggregate string concatenation with the above technique is not guaranteed. The actual behavior is plan-dependent so you may not get the desired results.
In SQL Server 2017 and Azure SQL Database, STRING_AGG is the proper method:
SELECT STRING_AGG(ColumnName, ',') WITHIN GROUP(ORDER BY ColumnOrder)
FROM #TempColumnsOrder;
In older SQL Versions like SQL Server 2012, the best method is with XML PATH():
SELECT #script = #script +
STUFF((SELECT ',' + ColumnName
FROM #TempColumnsOrder
ORDER BY ColumnOrder
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'),1,1,'');
See this answer for details about how the above query works.