SQL Concat field from multiple rows


I would like to create a function that returns a concatinated string of a given field of given query. Here is what I did.
And this one gives me an error.
Must declare the table variable "@qry".

  CREATE FUNCTION dbo.testing 
    @qry varchar(1000),
    @fld varchar(100),
    @separator varchar(15) = '; '
RETURNS  varchar
    DECLARE @rslt varchar(1000)
    SET @rslt ='' 

     SELECT @rslt = @rslt + @separator + CAST(@fld as varchar(160)) FROM  @qry

    RETURN @rslt

What I am trying to do is pass a query to this function and receive a concatinated string for certain field of the query.

Is this possible?

What am I doing wrong?

EDIT: BTW I have MSSQL Server 2005;

Best Solution

If you want to pass through any form of dynamic SQL, you need to execute it via EXEC or (preferred) sp_ExecuteSQL. Make sure your code's not subject to any injection attacks if you're using dynamic SQL as well lest you suffer the wrath of little Bobby Tables :-)