I found a page on techonthenet.com that explained an Oracle sequence better than I could have, so I plagerized it. Below is what it said. I think it makes it clear that a sequence object maintains a unique number but is not tied to a table entitity. Yes, in practice, you normally create a sequence for a specific table, and you then only use it on that one table, but there is nothing about the sequence that prevents you from using it anytime you want. You can even just "SELECT supplier_seq.nextval" to increment the counter arbitrarily.
Quoted from techonthenet.com:
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In Oracle, you can create an autonumber field by using sequences. A sequence is an object in Oracle that is used to generate a number sequence. This can be useful when you need to create a unique number to act as a primary key.
The syntax for a sequence is:
CREATE SEQUENCE sequence_name
MINVALUE value
MAXVALUE value
START WITH value
INCREMENT BY value
CACHE value;
For example:
CREATE SEQUENCE supplier_seq
MINVALUE 1
MAXVALUE 999999999999999999999999999
START WITH 1
INCREMENT BY 1
CACHE 20;
This would create a sequence object called
supplier_seq. The first sequence number that it would use is 1 and each subsequent number would increment by 1 (ie: 2,3,4,...}. It will cache up to 20 values for performance.
Now that you've created a sequence object to simulate an autonumber field, we'll cover how to retrieve a value from this sequence object. To retrieve the next value in the sequence order, you need to use
nextval.
For example:
supplier_seq.nextval
This would retrieve the next value from
supplier_seq. The
nextval statement needs to be used in an SQL statement. For example:
INSERT INTO suppliers
(supplier_id, supplier_name)
VALUES
(supplier_seq.nextval, 'Kraft Foods');
This insert statement would insert a new record into the
suppliers table. The
supplier_id field would be assigned the next number from the
supplier_seq sequence. The
supplier_name field would be set to Kraft Foods.
