How to include a variable data into a SQL Statement - sql-server

Question

I would like to know, how to include a variable as a part of a SQL statement, to clarify more about my question, here is the code
declare #PartNo as nvarchar(20)
declare #PPFno as nvarchar(20)
declare #Dimension as nvarchar(30)
declare #cursor CURSOR
declare #colname as nvarchar(30)
declare #top as integer
declare #query as nvarchar(MAX)
declare #categoryid NVARCHAR(MAX)
set #cursor = CURSOR FOR
(select [Name] from sys.columns where object_id = (select object_id from sys.tables where name = 'ProductProperty') and [Name] like 'T%' and [name] <> 'TEMP')order by [Name] asc
OPEN #cursor
FETCH NEXT
FROM #cursor INTO #colname
WHILE ##FETCH_STATUS = 0
BEGIN
--set #query = (select #colname from ProductProperty where PartNo = #PartNo and PPFNo = #PPFno )
--set #query = 'select distinct '+#colname+' from ProductProperty where PartNo = '''+#PartNo+''' and PPFNo = '''+#PPFno+''' and DName = '''+#Dimension+''''
BEGIN
EXEC sp_executesql N'set #categoryid = (select distinct #colname from ProductProperty where PartNo = #PartNo and PPFNo = #PPFno and DName = #Dimension)',
N'#colname nvarchar(30), #PartNo nvarchar(20), #PPFno nvarchar(20),#Dimension nvarchar(30), #categoryid NVARCHAR(MAX) OUTPUT', #colname,#PartNo, #PPFno,#Dimension, #categoryid OUTPUT
select #categoryid,#colname ,#PartNo
END
--end
FETCH NEXT
FROM #cursor INTO #colName
END
CLOSE #cursor
DEALLOCATE #cursor
Please do take note I did not included the variable types. I would just want to know how can #colname become part of the SQL Statement.
To Elaborate more. Using this code, I am receiving this data
Where T1 is the table name. I want to create a query where I could pass table names into a variable, then retrieve the contents of that query.
So the SQL Query should look like this:
select T1 from ProductProperty
But I am not receiving the query, instead, I am receiving the variable data, which is in the screenshot above.
The problem is, if you might notice in my code, I have the variable #categoryid as a output parameter. This is to check the contents of the query.
it seems like I am producing a query which looks like this
select 'T1' from ProductProperty
May I ask, what am I doing wrong? If you would want additional information, please do tell me.
EDIT:
completed the query for more clarification

Answer 1

The issue here is that you are telling it to select the value of the #colname variable itself as the first value in your query, not the value of the column name stored in the variable. This is the equivalent of doing:
SELECT #colname;
What you need to do is output the value of the #colname variable into the SQL string you are passing to sp_executesql. As long as the #colname variable isn't entered by the user, you can concatenate it into the string passed to sp_executesql, as follows:
DECLARE #SqlQuery NVARCHAR(MAX);
SET #SqlQuery = N'set #categoryid = (select distinct ' + #colname + ' from ProductProperty
where PartNo = #PartNo and PPFNo = #PPFno and DName = #Dimension)';
EXEC sp_executesql #SqlQuery,
#PartNo nvarchar(20), #PPFno nvarchar(20),#Dimension
nvarchar(30), #categoryid NVARCHAR(MAX) OUTPUT', #PartNo, #PPFno,#Dimension,
#categoryid OUTPUT
select #categoryid, #colname, #PartNo
That will effectively give you:
EXEC sp_executesql N'set #categoryid = (select distinct T1 from ProductProperty
where PartNo = #PartNo and PPFNo = #PPFno and DName = #Dimension)',
#PartNo nvarchar(20), #PPFno nvarchar(20),#Dimension
nvarchar(30), #categoryid NVARCHAR(MAX) OUTPUT', #PartNo, #PPFno,#Dimension,
#categoryid OUTPUT