Sequence value missing

Question

Hello Gurus,
I'm loading data from staging table to target and need to generate sequence while loading target. Using below tables.
Table 1 SRC (staging table) Table 2 : TGT ( target table)
ID name Sal           Seq ID name Sal
10 ‘abc’ 200 1 10 ‘abc’ 200
20 ‘sdf’ 300
30 'ghr' 400
40 'ghk' 500
My query is :
select t.ID,t.name,t.sal,decode(t.seq,0,SEQUENCE.nextval,1,NULL) as seq
from
(select src.ID ,src.name ,src.sal ,decode(tgt.seq,NULL,0,1) seq
from SRC
left outer join TGT
on src.id = tgt.id ) t
1st RUN :
ID name Sal seq
10 ‘abc’ 200 NULL
20 ‘sdf’ 300 1
30 'ghr' 400 2
40 'ghk' 500 *3*
2nd RUN :
ID name Sal seq
10 ‘abc’ 200 NULL
20 ‘sdf’ 300 *5*
30 'ghr' 400 6
40 'ghk' 500 7
Every time i execute the same query there is gap in sequence ( after 3 , 4 is missing. ).
Please let me know what im missing here.
or how to avoid the gap in sequence.
Using Oracle 10g Release 10.2.0.1.0
Thanks !

Answer 1

My query is :
select t.ID,t.name,t.sal,decode(t.seq,0,SEQUENCE.nextval,1,NULL) as seq
from
(select src.ID ,src.name ,src.sal ,decode(tgt.seq,NULL,0,1) seq
from SRC
left outer join TGT
on src.id = tgt.id ) tTake a look...
DECODE (t.seq,  0, SEQUENCE.NEXTVAL,  1, NULL)When it executes, whenever t.seq is 0 or 1, SEQUENCE.NEXTVAL executes and sequene is incrementing...
SQL> select test_seq.nextval from dual;
   NEXTVAL
         1
SQL> select decode(1,1,test_seq.nextval,2,2) from dual;
DECODE(1,1,TEST_SEQ.NEXTVAL,2,2)
                               2
SQL> select decode(1,1,test_seq.nextval,2,2) from dual;
DECODE(1,1,TEST_SEQ.NEXTVAL,2,2)
                               3
SQL> select decode(1,1,test_seq.nextval,2,2) from dual;
DECODE(1,1,TEST_SEQ.NEXTVAL,2,2)
                               4
SQL> select decode(2,1,test_seq.nextval,2,2) from dual;
DECODE(2,1,TEST_SEQ.NEXTVAL,2,2)
                               2
SQL> select decode(2,1,test_seq.nextval,2,2) from dual;
DECODE(2,1,TEST_SEQ.NEXTVAL,2,2)
                               2
SQL> select decode(1,1,test_seq.nextval,2,2) from dual;
DECODE(1,1,TEST_SEQ.NEXTVAL,2,2)
                               7