|Oracle9i SQL Reference
Release 2 (9.2)
Part Number A96540-01
SQL Statements: CREATE LIBRARY to CREATE SPFILE, 15 of 16
SEQUENCE statement to create a sequence, which is a database object from which multiple users may generate unique integers. You can use sequences to automatically generate primary key values.
When a sequence number is generated, the sequence is incremented, independent of the transaction committing or rolling back. If two users concurrently increment the same sequence, the sequence numbers each user acquires may have gaps because sequence numbers are being generated by the other user. One user can never acquire the sequence number generated by another user. Once a sequence value is generated by one user, that user can continue to access that value regardless of whether the sequence is incremented by another user.
Sequence numbers are generated independently of tables, so the same sequence can be used for one or for multiple tables. It is possible that individual sequence numbers will appear to be skipped, because they were generated and used in a transaction that ultimately rolled back. Additionally, a single user may not realize that other users are drawing from the same sequence.
Once a sequence is created, you can access its values in SQL statements with the
CURRVAL pseudocolumn (which returns the current value of the sequence) or the
NEXTVAL pseudocolumn (which increments the sequence and returns the new value).
To create a sequence in your own schema, you must have
To create a sequence in another user's schema, you must have
Specify the schema to contain the sequence. If you omit
schema, Oracle creates the sequence in your own schema.
Specify the name of the sequence to be created.
If you specify none of the following clauses, you create an ascending sequence that starts with 1 and increases by 1 with no upper limit. Specifying only
BY -1 creates a descending sequence that starts with -1 and decreases with no lower limit.
MAXVALUEparameter or specify
NOMAXVALUE. For descending sequences, omit the
MINVALUEparameter or specify the
MAXVALUEparameter. For a descending sequence, specify a value for the
MINVALUEparameter. Also specify the
NOCYCLE. Any attempt to generate a sequence number once the sequence has reached its limit results in an error.
MINVALUEparameters. Also specify the
CYCLE. If you do not specify
MINVALUE, then it defaults to
NOMINVALUE(that is, the value 1).
Specify the interval between sequence numbers. This integer value can be any positive or negative integer, but it cannot be 0. This value can have 28 or fewer digits. The absolute of this value must be less than the difference of
MINVALUE. If this value is negative, then the sequence descends. If the increment is positive, then the sequence ascends. If you omit this clause, the interval defaults to 1.
Specify the first sequence number to be generated. Use this clause to start an ascending sequence at a value greater than its minimum or to start a descending sequence at a value less than its maximum. For ascending sequences, the default value is the minimum value of the sequence. For descending sequences, the default value is the maximum value of the sequence. This integer value can have 28 or fewer digits.
This value is not necessarily the value to which an ascending cycling sequence cycles after reaching its maximum or minimum value.
Specify the maximum value the sequence can generate. This integer value can have 28 or fewer digits.
MAXVALUE must be equal to or greater than
WITH and must be greater than
NOMAXVALUE to indicate a maximum value of 1027 for an ascending sequence or -1 for a descending sequence. This is the default.
Specify the minimum value of the sequence. This integer value can have 28 or fewer digits.
MINVALUE must be less than or equal to
WITH and must be less than
NOMINVALUE to indicate a minimum value of 1 for an ascending sequence or -1026 for a descending sequence. This is the default.
CYCLE to indicate that the sequence continues to generate values after reaching either its maximum or minimum value. After an ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum.
NOCYCLE to indicate that the sequence cannot generate more values after reaching its maximum or minimum value. This is the default.
Specify how many values of the sequence Oracle preallocates and keeps in memory for faster access. This integer value can have 28 or fewer digits. The minimum value for this parameter is 2. For sequences that cycle, this value must be less than the number of values in the cycle. You cannot cache more values than will fit in a given cycle of sequence numbers. Therefore, the maximum value allowed for
CACHE must be less than the value determined by the following formula:
If a system failure occurs, all cached sequence values that have not been used in committed DML statements are lost. The potential number of lost values is equal to the value of the
Oracle Corporation recommends using the
NOCACHE to indicate that values of the sequence are not preallocated.
If you omit both
NOCACHE, Oracle caches 20 sequence numbers by default.
ORDER to guarantee that sequence numbers are generated in order of request. You may want to use this clause if you are using the sequence numbers as timestamps. Guaranteeing order is usually not important for sequences used to generate primary keys.
ORDER is necessary only to guarantee ordered generation if you are using Oracle with Real Application Clusters. If you are using exclusive mode, sequence numbers are always generated in order.
NOORDER if you do not want to guarantee sequence numbers are generated in order of request. This is the default.
The following statement creates the sequence
customers_seq in the sample schema
oe. This sequence could be used to provide customer ID numbers when rows are added to the
The first reference to
customers_seq.nextval returns 1000. The second returns 1001. Each subsequent reference will return a value 1 greater than the previous reference.