INSERT TB1 
SELECT NID,
       (SELECT MAX(FNUM) FROM TB1)+
       (SELECT COUNT(1) FROM TB2 WHERE NID<=T.NID) AS  FNUM ,FDES FROM TB2 T

------解决方案--------------------

SQL code
declare @a table(id int, fnum int, ftest char(10))

insert into @a select 1, 2, 'aa'
union select 21, 3, 'bb'
union select 33, 4, 'cc'

declare @b table(Nid int, Findex int, fdes char(10))
insert into @b select 11, 12, 'kk'
union select 22, 22, 'mm'
union select 33, 32, 'ss'

insert into @a(id, ftest, fnum) select Nid, fdes, ROW_NUMBER() OVER (order by Findex) + (select MAX(fnum) from @a)  from @b as B

select * from @a