Dynamic SQL error converting nvarchar to int - sql

Question

I have created a procedure in dynamic SQL which has a select statement and the code looks like:
ALTER PROCEDURE cagroup (
#DataID INT ,
#days INT ,
#GName VARCHAR(50) ,
#T_ID INT ,
#Act BIT ,
#Key VARBINARY(16)
)
AS
BEGIN
DECLARE #SQL NVARCHAR(MAX)
DECLARE #SchemaName SYSNAME
DECLARE #TableName SYSNAME
DECLARE #DatabaseName SYSNAME
DECLARE #BR CHAR(2)
SET #BR = CHAR(13) + CHAR(10)
SELECT #SchemaName = Source_Schema ,
#TableName = Source_Table ,
#DatabaseName = Source_Database
FROM Source
WHERE ID = #DataID
SET #SQL = 'SELECT ' + #GName + ' AS GrName ,' + #BR
+ #T_ID + ' AS To_ID ,' + #BR
+ #DataID + ' AS DataSoID ,' + #BR
+ #Act + ' AS Active ,' + #BR
+ Key + ' AS key' + #BR
+ 'R_ID AS S_R_ID' + #BR
+ 'FROM' + #DatabaseName + '.'
+ #SchemaName + '.'
+ #TableName + ' t' + #BR
+ 'LEFT OUTER JOIN Gro g ON g.GName = '
+ #GName + #BR + 'AND g.Data_ID] =' + #DataID + #BR
+ 't.[I_DATE] > GETDATE() -' + #days + #BR
+ 'g.GName IS NULL
AND ' + #GName + ' IS NOT NULL
AND t.[Act] = 1' + #BR
PRINT (#SQL)
END
When I am executing this procedure with this statement:
Exec dbo.cagroup 1,10,'[Gro]',1,1,NULL
I am getting the following error.
Msg 245, Level 16, State 1, Procedurecagroup, Line 33
Conversion failed when converting the nvarchar value 'SELECT [Gro] AS GName ,
' to data type int.
Where am I doing wrong?

Answer 1

You need to CAST all numbers to nvarchar in the concatenation.
There is no implicit VBA style conversion to string. In SQL Server data type precedence means ints are higher then nvarchar: so the whole string is trying to be CAST to int.
SET #SQL = 'SELECT ' + #GName + ' AS GrName ,' + #BR
+ CAST(#T_ID AS nvarchar(10)) + ' AS To_ID ,' ...
Edit: Will A has a good point: watch for NULLs!

Answer 2

If you have to build this kind of dynamic SQL, it is better to get the column information from the meta-data than to pass it around.
Select * from Information_Schema.Columns Where Table_name=#TableName
The you have to write an ugly cursor to build the SQL. Expect performance problems. I do lots of this during development to write code for me, but I don't dare run it in production.