Nextval vs currval

Question

Hello,
Can someone please explain to me the following behavior:
select
OBJECT_ID,
MOD(OBJECT_ID,2),
CASE WHEN MOD(OBJECT_ID,2)=1 then seq_general.nextval else seq_general.currval END
from user_OBJECTS;
returns something like:
OBJ_ID, MOD, SEQ
1,1,1
2,0,2
3,1,3
4,0,4
when instead I was aiming for:
1,1,1
2,0,1
3,1,2
4,0,2
Thank you

Answer 1

Hi,
The explanation is easy, see [How to Use Sequence Values|http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96540/sql_elements6a.htm#80564]:
Within a single SQL statement containing a reference to NEXTVAL, Oracle increments the sequence only once:
* For each row returned by the outer query block of a SELECT statement. Such a query block can appear in the following places:
o A top-level SELECT statement
o An INSERT ... SELECT statement (either single-table or multi-table). For a multi-table insert, the reference to NEXTVAL must appear in the VALUES clause, and the sequence is updated once for each row returned by the subquery, even though NEXTVAL may be referenced in multiple branches of the multi-table insert.
o A CREATE TABLE ... AS SELECT statement
o A CREATE MATERIALIZED VIEW ... AS SELECT statement
* For each row updated in an UPDATE statement
* For each INSERT statement containing a VALUES clause
* For row "merged" (either inserted or updated) in a MERGE statement. The reference to NEXTVAL can appear in the merge_insert_clause or the merge_update_clause.
If any of these locations contains more than one reference to NEXTVAL, then Oracle increments the sequence once and returns the same value for all occurrences of NEXTVAL.
If any of these locations contains references to both CURRVAL and NEXTVAL, then Oracle increments the sequence and returns the same value for both CURRVAL and NEXTVAL.
Also see a very good explanation in [Why is output of select seqname.currval,seqname.nextval,seqname.currval,seqname.nextval from dual like this?|http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:21115948632804]
Regards,
Edited by: Walter Fernández on Jul 7, 2009 7:45 PM