SQL中的union使用方法

Union运算符，可以将多个select查询结果集并成为一个结果集合。

案例赏析：    Sqltxt = " Select Property_Num,isnull(Replace(Property_Num,' ',''),'')+'-'+isnull(Replace(Property_Name,' ',''),'') as Property_Name from T_Cod_Property where Property_Type='ControlLevel' " +
                 " union " +
                 " Select '0' Property_Num,'全部' as Property_Name from T_Cod_Property" +
                 " Order by Property_Num ";