CREATE SEQUENCE creates a new sequence number generator. This involves creating and initializing a new special single-row table with the name, name
. The generator will be owned by the user issuing the command.
The optional clause INCREMENT BY increment specifies the value to add to the current sequence value to create a new value. A positive value will make an ascending sequence, a negative one a descending sequence. The default value is 1.
The optional clause MINVALUE minvalue determines the minimum value a sequence can generate. If this clause is not supplied, then defaults will be used. The defaults are 1 and -263-1 for ascending and descending sequences, respectively. Note that the key words, NOMINVALUE, may be used to set this behavior to the default.
The optional clause MAXVALUE maxvalue determines the maximum value for the sequence. If this clause is not supplied, then default values will be used. The defaults are 263-1 and -1 for ascending and descending sequences, respectively. Note that the key words, NOMAXVALUE, may be used to set this behavior to the default.
The optional clause START WITH start allows the sequence to begin anywhere. The default starting value is minvalue for ascending sequences and maxvalue for descending ones.
The optional clause CACHE cache specifies how many sequence numbers are to be preallocated and stored in memory for faster access. The minimum value is 1 (only one value can be generated at a time, i.e., NOCACHE), and this is also the default.
The CYCLE option allows the sequence to wrap around when the maxvalue or minvalue has been reached by an ascending or descending sequence respectively. If the limit is reached, the next number generated will be the minvalue or maxvalue
is omitted (the default), any calls to NEXTVAL
after the sequence has reached its maximum value will return an error. Note that the key words, NO CYCLE
, may be used to obtain the default behavior, however, this term is not compatible with Oracle databases
Unexpected results may be obtained if a cache
setting greater than one is used for a sequence object that will be used concurrently by multiple sessions. Each session will allocate and cache successive sequence values during one access to the sequence object and increase the sequence object’s last value accordingly. Then, the next cache
-1 uses of NEXTVAL
within that session simply return the preallocated values without touching the sequence object. So, any numbers allocated but not used within a session will be lost when that session ends, resulting in "holes" in the sequence.