Using variables within a dynamic sql script - sql-server

Question

I'm currently editing a procedure, let's call it A, that does some manipulations of a temporary table created in procedure B (so procedure B calls A). Procedure B contains one argument, which is the name of the said temporary table.
That being said, we need to retrieve all the columns (in a CSV) of the temporary table into a variable. Since the name is not the same everytime, I need to use dynamic SQL.
Here is what I have:
-- Input parameters: #sTempTableName VARCHAR(MAX) --> Name of the temporary table to alter
DECLARE
#sColumns AS NVARCHAR(MAX) = '',
#sAlterTableDynamicSQL AS VARCHAR(MAX)
SET #sAlterTableDynamicSQL =
'
SELECT #sColumns = #sColumns + [name] + N'','' FROM Tempdb.Sys.Columns WHERE [object_id] = object_id(''tempdb..' + #sTempTableName + ''')
'
PRINT (#sAlterTableDynamicSQL)
EXEC (#sAlterTableDynamicSQL)
This fails saying that I need to declare #sColumns since it does not exists (I guess variables aren't shared between connections). How can I manage to do this ? I figured I could create another temporary table to hold that CSV, but I'm thinking there should be another way.

Answer 1

You should use sp_executesql statement. It would look like this:
DECLARE
#sColumns NVARCHAR(MAX),
#sAlterTableDynamicSQL NVARCHAR(MAX),
#temptableid INT = 3;
SELECT #sAlterTableDynamicSQL =
N'SELECT #sColumns = STRING_AGG([name], '','') FROM Tempdb.Sys.Columns WHERE [object_id] = + CAST(#temptableid AS VARCHAR(10))';
EXEC sp_executesql #sAlterTableDynamicSQL, N'#sColumns NVARCHAR(MAX) OUTPUT, #temptableid INT', #sColumns = #sColumns OUTPUT, #temptableid = #temptableid
SELECT #sColumns;

Answer 2

You can pass through variables to dynamic SQL via sp_executesql
Note that you should always use QUOTENAME to escape object names
Also, dynamic SQL variables should always be nvarchar
You also should not use variable coalescing to aggregate, instead use STRING_AGG or FOR XML
DECLARE
#sColumns AS NVARCHAR(MAX) = '',
#sAlterTableDynamicSQL AS NVARCHAR(MAX),
#sGUID AS VARCHAR(MAX) = CAST(NEWID() AS VARCHAR(MAX))
SET #sAlterTableDynamicSQL =
'
SELECT #sColumns = STRING_AGG(CAST([name] AS nvarchar(max)), N'','')
FROM Tempdb.sys.columns
WHERE [object_id] = object_id(N''tempdb..' + QUOTENAME(#sNomTableTemporaire, '''') + ''');
';
PRINT (#sAlterTableDynamicSQL);
EXEC sp_executesql
#sAlterTableDynamicSQL,
N'#sColumns nvarchar(max) OUTPUT'
#sColumns = #sColumns OUTPUT;
But you don't actually need dynamic SQL here at all. You can pass the table name straight to object_id()
DECLARE
#sColumns AS NVARCHAR(MAX) = '',
#sAlterTableDynamicSQL AS NVARCHAR(MAX),
#sGUID AS VARCHAR(MAX) = CAST(NEWID() AS VARCHAR(MAX))
SELECT #sColumns = STRING_AGG(CAST([name] AS nvarchar(max)), N',')
FROM Tempdb.sys.columns
WHERE [object_id] = object_id(N'tempdb..' + QUOTENAME(#sNomTableTemporaire));
For SQL Server 2016 and earlier, you can use the FOR XML PATH('') method