The ALTER SEQUENCE statement in SQL is used to modify an existing sequence object. A sequence is a database object that generates a sequence of unique numeric values, often used for primary keys.
Here's the general syntax for using the ALTER SEQUENCE statement:
ALTER SEQUENCE sequence_name
[ INCREMENT BY increment_value ]
[ MINVALUE min_value | NO MINVALUE ]
[ MAXVALUE max_value | NO MAXVALUE ]
[ START WITH start_value ]
[ CYCLE | NO CYCLE ]
[ CACHE cache_value | NO CACHE ];
Components of the Statement:
- sequence_name: The name of the sequence you want to modify.
- INCREMENT BY: Specifies the value by which the sequence will be incremented each time a new value is generated.
- MINVALUE: Specifies the minimum value the sequence can have. If you specify NO MINVALUE, it removes the minimum value limitation.
- MAXVALUE: Specifies the maximum value the sequence can have. If you specify NO MAXVALUE, it removes the maximum value limitation.
- START WITH: Specifies the first value to be generated by the sequence.
- CYCLE: If specified, the sequence will restart from the minimum (or maximum) value when the limit is reached. If NO CYCLE is specified, the sequence will not wrap around.
- CACHE: Specifies how many sequence numbers to preallocate and cache in memory for faster access. NO CACHE means no numbers are cached.
Example:
Here’s a simple example of modifying a sequence:
ALTER SEQUENCE my_sequence
INCREMENT BY 5
MINVALUE 1
MAXVALUE 1000
CYCLE;
In this example:
- The increment value for my_sequence is changed to 5.
- The minimum value is set to 1.
- The maximum value is set to 1000.
- The sequence will cycle back to the minimum value when it reaches its maximum value.
Important Notes:
- When you modify a sequence, existing values that have already been generated are not affected. However, the next value that will be generated will change according to the adjustments made.
- Make sure to only use these changes that are appropriate for your application to prevent any unintended behavior, especially in cases where sequences are closely tied to primary key generation.