How to decrease sequence value in Oracle?

Oracle sequences are commonly used to generate unique numbers. However, there may be situations where you need to decrease the value of a sequence. Although Oracle does not provide a direct method to decrease the value of a sequence, there are alternative approaches that you can take.

Using negative values as increments/decrements

One way to decrease the value of a sequence in Oracle is by using negative values as increments or decrements. By default, a sequence increments by 1, but you can alter the sequence to use a negative increment to decrease the value.

To do this, you can use the `INCREMENT BY` clause to set a negative value as the increment:

ALTER SEQUENCE my_sequence INCREMENT BY -1;

Once you have set the negative increment, you can fetch the next sequence value with the following query:

SELECT my_sequence.NEXTVAL FROM dual;

This will decrement the value of the sequence by 1.

Avoiding negative values if sequence cycling is enabled

If you are using cycling in your sequence, it’s important to avoid negative values, as it might lead to unexpected results. Cycling allows the sequence to start again from the minimum value after it reaches the maximum value or vice versa. In such scenarios, using negative increments may not work as intended.

To disable cycling and safely decrease the value of a sequence using negative increments, you can use the following command:

ALTER SEQUENCE my_sequence NOCYCLE;

Make sure to re-enable cycling once you are done decreasing the sequence value:

ALTER SEQUENCE my_sequence CYCLE;

Resetting a sequence

Alternatively, if it is acceptable to reset the sequence to a lower value, you can use the `DROP` and `CREATE` commands to achieve this. However, keep in mind that resetting a sequence will cause gaps in the sequence values and may affect any other database entities that rely on the sequence.

Here is an example of how to reset a sequence:


-- Backup the current sequence
CREATE SEQUENCE my_sequence_backup START WITH 0;

-- Recreate the sequence with a lower start value
CREATE SEQUENCE my_sequence START WITH 100;


This will reset the sequence to a lower value (in this case, 100) and any subsequent calls to `NEXTVAL` will return the decreased value.

Related FAQs:

1. Can you decrease the value of a sequence in Oracle without any side effects?

No, decreasing the value of a sequence may cause gaps in the sequence values and potentially affect entities that rely on the sequence.

2. What happens if I set a negative increment on a sequence with cycling enabled?

If cycling is enabled and you use negative increments, the sequence may not behave as intended and cycling may override the negative increments.

3. Can I decrease the value of a sequence to a specific value?

Yes, by resetting the sequence using the `DROP` and `CREATE` commands, you can set the sequence to any specific value.

4. How can I check the current value of a sequence in Oracle?

To check the current value of a sequence, you can use the following query: `SELECT my_sequence.CURRVAL FROM dual;`

5. Is it possible to decrease a sequence by more than one at a time?

No, by default, sequences in Oracle increment or decrement by one. If you need to decrease by more than one, you must execute the decrement multiple times.

6. Can I decrease a sequence value using PL/SQL?

Yes, you can use PL/SQL to execute the necessary commands to decrease a sequence value. Use the appropriate SQL statements within the PL/SQL block.

7. How can I prevent gaps in the sequence values when decreasing a sequence?

Decreasing a sequence value will inherently cause gaps. If you want to prevent gaps, consider resetting the sequence instead, but be aware of the potential implications.

8. What happens if I decrease the value of a sequence to a value lower than the current minimum?

If you try to decrease the value of a sequence to a value lower than the current minimum, Oracle will raise an error.

9. Is it possible to decrease the value of a sequence without altering it?

No, altering the sequence is necessary to decrease its value. However, you can always reset the sequence as an alternative approach.

10. Can I decrease the value of a sequence that is used by other database objects?

Yes, you can decrease the value of a sequence even if it is used by other database objects. However, altering the sequence may affect the objects that rely on it, so proceed with caution.

11. How can I increment the value of a sequence by a fixed value?

By default, sequences increment by one. If you want to increase by a fixed value, you can alter the sequence to use a positive increment higher than one.

12. What is the purpose of using sequences in Oracle?

Sequences are primarily used to generate unique numbers for primary keys or other situations where unique identifiers are required. They ensure uniqueness and provide an efficient way to generate values.

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