Sequence Pseudocolumns

A sequence is a schema object that can generate unique sequential values. These values are often used for primary and unique keys. You can refer to sequence values in SQL statements with these pseudocolumns:

CURRVAL: Returns the current value of a sequence
NEXTVAL: Increments the sequence and returns the next value

You must qualify CURRVAL and NEXTVAL with the name of the sequence:

sequence.CURRVAL
sequence.NEXTVAL

To refer to the current or next value of a sequence in the schema of another user, you must have been granted either SELECT object privilege on the sequence or SELECT ANY SEQUENCE system privilege, and you must qualify the sequence with the schema containing it:

schema.sequence.CURRVAL
schema.sequence.NEXTVAL

To refer to the value of a sequence on a remote database, you must qualify the sequence with a complete or partial name of a database link:

schema.sequence.CURRVAL@dblink
schema.sequence.NEXTVAL@dblink

A sequence can be accessed by many users concurrently with no waiting or locking.

See Also: References to Objects in Remote Databases for more information on referring to database links

Where to Use Sequence Values

You can use CURRVAL and NEXTVAL in the following locations:

The select list of a SELECT statement that is not contained in a subquery, materialized view, or view
The select list of a subquery in an INSERT statement
The VALUES clause of an INSERT statement
The SET clause of an UPDATE statement

Restrictions on Sequence Values

You cannot use CURRVAL and NEXTVAL in the following constructs:

A subquery in a DELETE, SELECT, or UPDATE statement
A query of a view or of a materialized view
A SELECT statement with the DISTINCT operator
A SELECT statement with a GROUP BY clause or ORDER BY clause
A SELECT statement that is combined with another SELECT statement with the UNION, INTERSECT, or MINUS set operator
The WHERE clause of a SELECT statement
The condition of a CHECK constraint

Within a single SQL statement that uses CURRVAL or NEXTVAL, all referenced LONG columns, updated tables, and locked tables must be located on the same database.

How to Use Sequence Values

When you create a sequence, you can define its initial value and the increment between its values. The first reference to NEXTVAL returns the initial value of the sequence. Subsequent references to NEXTVAL increment the sequence value by the defined increment and return the new value. Any reference to CURRVAL always returns the current value of the sequence, which is the value returned by the last reference to NEXTVAL.

Before you use CURRVAL for a sequence in your session, you must first initialize the sequence with NEXTVAL. Refer to CREATE SEQUENCE for information on sequences.

Within a single SQL statement containing a reference to NEXTVAL, Oracle increments the sequence once:

For each row returned by the outer query block of a SELECT statement. Such a query block can appear in the following places:
- A top-level SELECT statement
- An INSERT ... SELECT statement (either single-table or multitable). For a multitable 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 multitable insert.
- A CREATE TABLE ... AS SELECT statement
- A CREATE MATERIALIZED VIEW ... AS SELECT statement
For each row updated in an UPDATE statement
For each INSERT statement containing a VALUES clause
For each INSERT ... [ALL \ FIRST] statement (multitable insert). A multitable insert is considered a single SQL statement. Therefore, a reference to the NEXTVAL of a sequence will increase the sequence only once for each input record coming from the SELECT portion of the statement. If NEXTVAL is specified more than once in any part of the INSERT ... [ALL \ FIRST ] statement, then the value will be the same for all insert branches, regardless of how often a given record might be inserted.
For each row merged by a MERGE statement. The reference to NEXTVAL can appear in the merge_insert_clause or the merge_update_clause or both. The NEXTVALUE value is incremented for each row updated and for each row inserted, even if the sequence number is not actually used in the update or insert operation. If NEXTVAL is specified more than once in any of these locations, then the sequence is incremented once for each row and returns the same value for all occurrences of NEXTVAL for that row.
For each input row in a multitable INSERT ALL statement. NEXTVAL is incremented once for each row returned by the subquery, regardless of how many occurrences of the insert_into_clause map to each row.

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.

Finding the next value of a sequence: Example

This example selects the next value of the employee sequence in the sample schema hr:

SELECT employees_seq.nextval
  FROM DUAL;

Inserting sequence values into a table: Example

This example increments the employee sequence and uses its value for a new employee inserted into the sample table hr.employees:

INSERT INTO employees
  VALUES (employees_seq.nextval, 'John', 'Doe', 'jdoe', '555-1212',
          TO_DATE(SYSDATE), 'PU_CLERK', 2500, null, null, 30);

Reusing the current value of a sequence: Example

This example adds a new order with the next order number to the master order table. It then adds suborders with this number to the detail order table:

INSERT INTO orders (order_id, order_date, customer_id)
  VALUES (orders_seq.nextval, TO_DATE(SYSDATE), 106);

INSERT INTO order_items (order_id, line_item_id, product_id)
  VALUES (orders_seq.currval, 1, 2359);

INSERT INTO order_items (order_id, line_item_id, product_id)
  VALUES (orders_seq.currval, 2, 3290);

INSERT INTO order_items (order_id, line_item_id, product_id)
  VALUES (orders_seq.currval, 3, 2381);