declare @vip_sorder table(billid int,bomno varchar(2),materialid varchar(1),mqty int,elemgid varchar(1),dbomno varchar(2),dqty int)
insert into @vip_sorder
select 1 , 'AN' , 'A ', 1 , 'B' , 'BN' , 1 union all
select 1 , 'AN' , 'A' , 1 , 'C' , ' ', 1 union all
select 1 , 'AN' , 'A' , 1 , 'D' , ' ', 1 union all
select 2 , 'BN' , 'B' , 1 , 'E' , 'EN', 1 union all
select 2 , 'BN' , 'B' , 1 , 'F' , '' , 1 union all
select 3 , 'EN' , 'E' , 1 , 'G' , '' , 1 union all
select 4 , 'AK' , 'A' , 1 , 'B' , 'EK' , 1 union all
select 5 , 'EK' , 'B' , 1 , 'J' , '' , 1
select * from @vip_sorder

;with cte as
(
    select billid,bomno,materialid,mqty,elemgid,dbomno,dqty from @vip_sorder where materialid='A' and bomno in ('AN','AK')
    union all
    select cte.billid,cte.bomno,cte.materialid,t.mqty,t.elemgid,t.dbomno,t.dqty from @vip_sorder t,cte
    where cte.dbomno=t.bomno
)

select * from cte
where dbomno=''
order by billid,bomno,materialid
/*
billid      bomno materialid mqty        elemgid dbomno dqty
----------- ----- ---------- ----------- ------- ------ -----------
1           AN    A          1           C              1
1           AN    A          1           D              1
1           AN    A          1           F              1
1           AN    A          1           G              1
4           AK    A          1           J              1
*/