Microsoft SQL Server 2014 - 告诉我下面的SQL语句有什么问题

declare @UnitIdn nchar(30);

set @UnitIdn = N'b';

set @UnitIdn =  @UnitIdn + N'aaaaa';

select @UnitIdn


set @UnitIdn = N'b';

set @UnitIdn =  N'aaaaa' + @UnitIdn 

select @UnitIdn
declare @UnitIdn nchar(30);

set @UnitIdn = N'b