NEXTVAL FAQS

 1. What is NEXTVAL in Oracle?

NEXTVAL is a function used to generate the next value from a sequence in Oracle. A sequence generates unique numeric values, often used for generating primary key values in tables.

2. How do I create a sequence in Oracle?

You can create a sequence using the CREATE SEQUENCE statement. Here's an example:

CREATE SEQUENCE emp_seq

START WITH 1000

INCREMENT BY 1;

3. How do I retrieve the next value from a sequence?

You can retrieve the next value using the NEXTVAL function:

SELECT emp_seq.NEXTVAL FROM dual;

This will return the next value of the sequence emp_seq.

4. What is the difference between NEXTVAL and CURRVAL?

• NEXTVAL: Retrieves the next sequence value and increments the sequence.
• CURRVAL: Retrieves the current value of the sequence (the last value returned by NEXTVAL) but does not increment the sequence.

5. Can I use CURRVAL before calling NEXTVAL?

No, you must call NEXTVAL before using CURRVAL. If you try to use CURRVAL without calling NEXTVAL first, Oracle will throw an error.

Example:

SELECT emp_seq.CURRVAL FROM dual;  -- Error, must use NEXTVAL first

6. What happens if I call NEXTVAL multiple times in one query?

Each time you call NEXTVAL, the sequence is incremented. Oracle will return a different value for each NEXTVAL call.

Example:

SELECT emp_seq.NEXTVAL, emp_seq.NEXTVAL FROM dual;

This will return two different values, incremented by 1 each time.

7. Can I set a maximum value for a sequence?

Yes, you can set a MAXVALUE to limit the upper bound of the sequence. Once the sequence reaches this value, it will either stop or cycle back to the MINVALUE (if CYCLE is specified).

Example:

CREATE SEQUENCE emp_seq

START WITH 1

INCREMENT BY 1

MAXVALUE 100;

8. What is caching in sequences?

Caching is a feature that allows Oracle to pre-allocate a set of sequence values in memory for faster access. By using the CACHE option, you can reduce disk I/O by caching a number of sequence values.

Example:

CREATE SEQUENCE emp_seq

START WITH 1

INCREMENT BY 1

CACHE 10;

9. What happens if the sequence is cached and the system crashes?

If the system crashes before the cached sequence numbers are used, there will be gaps in the sequence. However, Oracle will continue from the cached value, meaning those numbers are lost and won’t be reused.

10. Can I use a sequence with a negative increment?

Yes, you can specify a negative increment when creating a sequence. This is useful when you want to generate descending sequence values.

Example:

CREATE SEQUENCE emp_seq

START WITH -1

INCREMENT BY -1;

11. Can a sequence cycle back to its starting value?

Yes, if you define the sequence with the CYCLE option, it will restart from the MINVALUE once it reaches the MAXVALUE.

Example:

CREATE SEQUENCE emp_seq

START WITH 1

INCREMENT BY 1

MAXVALUE 100

CYCLE;

12. What happens if a sequence reaches its maximum value?

If a sequence reaches its MAXVALUE and is not defined to cycle, it will raise an error on subsequent NEXTVAL calls. If the sequence is defined to cycle, it will start again from the MINVALUE.

13. Can I modify a sequence after it has been created?

You cannot modify a sequence directly (like changing its increment, start value, etc.) once it has been created. To change a sequence, you would have to drop and recreate it.

14. What happens when I roll back a transaction after using NEXTVAL?

Even if you roll back a transaction, the sequence value is still incremented. The sequence does not roll back as it is independent of transactions.

15. Can I use NEXTVAL in an INSERT statement?

Yes, you can use NEXTVAL to generate a value for an INSERT statement. For example:

INSERT INTO employees (employee_id, name)

VALUES (emp_seq.NEXTVAL, 'John Doe');

These are just a few common questions. Let me know if you'd like clarification on any of these points or need further examples!