If id is null, use the value from sequence : Value « Sequences « Oracle PL/SQL Tutorial

SQL>
SQL> create table myTable(
  2      id           number primary key,
  3      blob_content blob
  4  )
  5  /

Table created.

SQL>
SQL> create sequence myTable_seq
  2  /

Sequence created.

SQL>
SQL> create or replace trigger biu_myTable
  2      before insert or update on myTable
  3      for each row
  4  begin
  5      if :new.id is null then
  6          select myTable_seq.nextval into :new.id from dual;
  7      end if;
  8  end;
  9  /

Trigger created.

SQL> drop table myTable;

Table dropped.

SQL> drop sequence myTable_seq;

Sequence dropped.

5.6.Value
	5.6.1.	Set start value, mini/max value and cache
	5.6.2.	Once initialized, you can get the current value from the sequence using currval.
	5.6.3.	When you select currval , nextval remains unchanged; nextval only changes when you select nextval to get the next value.
	5.6.4.	Query current sequence value
	5.6.5.	Get next value from sequence
	5.6.6.	select deptno_seq.currval, deptno_seq.nextval
	5.6.7.	Automatically including unique sequence numbers during an INSERT.
	5.6.8.	Use dual table to check sequence
	5.6.9.	Random value based on sequence
	5.6.10.	If id is null, use the value from sequence