sql - Dynamic query is not giving desired result -

i creating dynamic query updating column if exists in table. when update query running giving error. following procedure

create procedure spr_updatecolumnwithtotalprice @id int begin  if not exists (     select * information_schema.columns     table_name = 'product'     ,     column_name = 'productsellingprice' ) begin     declare @sellingprice varchar(max)       set @sellingprice = 'alter table product add productsellingprice int'     exec(@sellingprice)      print 'table altered' end else begin         set @sellingprice = 'update product set productsellingprice = ((unitprice * [gstrate%]/100) + unitprice )  productid  = @id'         exec(@sellingprice)         print 'table updated' end 

end

i getting following result :-

conversion failed when converting varchar value 'update product set productsellingprice = ((unitprice * [gstrate%]/100) + unitprice ) productid = @id' data type int.

please , in advance

try instead:

set @sellingprice = 'update product set productsellingprice = ((unitprice * [gstrate%]/100) + unitprice )  productid  = '+ cast(@id varchar(13)) 

you can't use + concatenate int string, since sql server try implicitly convert string int.
why need explicitly convert int string.