SQL中一个不明的错误

ERROR>>未能找到 ID 为 104 的数据库。可能该数据库尚未激活，也可能正在转换过程中。
这个错误我是认为比较诡异了。请高手解释一下

if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[fn]') and xtype in (N'FN', N'IF', N'TF'))
drop function [dbo].[fn]
GO
CREATE FUNCTION  fn
(
    @type     varchar(32)
)
RETURNS varchar(32)
AS
Begin
    declare @Status_for  varchar(32)
    set @Status_for=case @type
                    when 'd'  then  'd'
                    when 'c'  then  'c'                   
                    else  'UnKnow'
                  end
    return @Status_for   
End
GO

create table #t
(
  col1     varchar(32),
  col2     varchar(32),
  col3     int
)
insert into #t(col1, col2, col3) values('#t1', 'd', 1)
insert into #t(col1, col2, col3) values('#t2', 'd', 2)
insert into #t(col1, col2, col3) values('#t3', 'c', 1)
insert into #t(col1, col2, col3) values('#t4', 'c', 2)

create table #b
(
  b1     int,
  b2     varchar(32),
  b3     varchar(32)
)
insert into #b(b1, b2, b3) values(1, 'd', 'd1')
insert into #b(b1, b2, b3) values(2, 'd', 'd2')
insert into #b(b1, b2, b3) values(1, 'c', 'c1')
insert into #b(b1, b2, b3) values(2, 'c', 'c2')

--下面的查询没有问题
select *
from       #t a
left join  #b b on b.b1 = a.col3 and b.b2 = dbo.fn(a.col2)

--这个子查询，错误就比较诡异了
select * from
(
select *
from       #t a
left join  #b b on b.b1 = a.col3 and b.b2 = dbo.fn(a.col2)
) a

--clear the garbage
drop table #t
drop table #b
drop function dbo.fn