The CREATE SEQUENCE statement creates a sequence at the current server.
This statement can be embedded in an application program or issued interactively. It is an executable statement that can be dynamically prepared only if DYNAMICRULES run behavior is implicitly or explicitly specified.
The privilege set that is defined below must include at least one of the following:
- The CREATEIN privilege on the schema
- SYSADM or SYSCTRL authority
- System DBADM
The authorization ID that matches the schema name implicitly has the CREATEIN privilege on the schema.
Privilege set: If the statement is embedded in an application program, the privilege set is the privileges that are held by the of the owner of the plan or package. If the application is bound in a trusted context with the ROLE AS OBJECT OWNER clause specified, a role is the owner. Otherwise, an authorization ID is the owner.
If the statement is dynamically prepared, the privilege set is the privileges that are held by the SQL authorization ID of the process unless the process is within a trusted context and the ROLE AS OBJECT OWNER clause is specified. In that case, the privileges set is the privileges that are held by the role that is associated with the primary authorization ID of the process.
If the data type of the sequence is a distinct type, the privilege set must include the USAGE privilege on the distinct type.
- 1 The same clause must not be specified more than once. Separator commas can be specified between sequence attributes when a sequence is defined.
- Names the sequence. The name, including
the implicit or explicit qualifiers, must not identify an existing
sequence at the current server, including the sequence names that
are generated by Db2.
The schema name must not begin with 'SYS' unless the schema name is 'SYSADM'.
- AS data-type
- Specifies the data type to be used for the sequence value. The data type can be any exact numeric data type (SMALLINT, INTEGER, BIGINT, or DECIMAL with a scale of zero), or a user-defined distinct type for which the source type is an exact numeric data type with a scale of zero. The default, when AS is not specified, is INTEGER. If DECIMAL is specified, the default is DECIMAL(5,0).
- START WITH numeric-constant
the first value for the sequence. The value can be any positive or
negative value that could be assigned to the a column of the data
type that is associated with the sequence without non-zero digits
existing to the right of the decimal point.
If the START WITH clause is not explicitly specified with a value, the default is the MINVALUE for ascending sequences and MAXVALUE for descending sequences.
This value is not necessarily the value that a sequence would cycle to after reaching the maximum or minimum value of the sequence. The START WITH clause can be used to start a sequence outside the range that is used for cycles. The range used for cycles is defined by MINVALUE and MAXVALUE.
- INCREMENT BY numeric-constant
the interval between consecutive values of the sequence. The value
can be any positive or negative value (including 0) that could be
assigned to a column of the data type that is associated with the
sequence without any non-zero digits existing to the right of the
decimal point. The default is 1.
If INCREMENT BY is positive, the sequence ascends. If INCREMENT BY is negative, the sequence descends. If INCREMENT is 0, the sequence is treated as an ascending sequence.
The absolute value of INCREMENT BY can be greater than the difference between MAXVALUE and MINVALUE.
- MINVALUE or NO MINVALUE
the minimum value at which a descending sequence either cycles or
stops generating values or an ascending sequence cycles to after reaching
the maximum value. The default is NO MINVALUE.
- MINVALUE numeric-constant
- Specifies the minimum end of the range of values for the sequence. The last value that is generated for a cycle of a descending sequence will be equal to or greater than this value. MINVALUE is the value to which an ascending sequence cycles to after reaching the maximum value.
The value can be any positive or negative value that could be assigned to the a column of the data type that is associated with the sequence without non-zero digits existing to the right of the decimal point. The value must be less than or equal to the maximum value.
For the effects of defining MINVALUE and MAXVALUE with the same value, see Defining a constant sequence.
- NO MINVALUE
- Specifies that the minimum end point of the range of values for
the sequence has not been specified explicitly. In such a case, the
default value for MINVALUE becomes one of the following:
- For an ascending sequence, the value is the START WITH value or 1 if START WITH is not specified.
- For a descending sequence, the value is the minimum value of the data type that is associated with the sequence.
- MAXVALUE or NO MAXVALUE
the maximum value at which an ascending sequence either cycles or
stops generating values or an descending sequence cycles to after
reaching the minimum value. The default is NO MAXVALUE.
- MAXVALUE numeric-constant
- Specifies the maximum end of the range of values for the sequence. The last value that is generated for a cycle of an ascending sequence will be less than or equal to this value. MAXVALUE is the value to which a descending sequence cycles to after reaching the minimum value.
The value can be any positive or negative value that could be assigned to the a column of the data type that is associated with the sequence without non-zero digits existing to the right of the decimal point. The value must be greater than or equal to the minimum value.
For the effects of defining MAXVALUE and MINVALUE with the same value, see Defining a constant sequence.
- NO MAXVALUE
- Specifies the maximum end point of the range of values for the
sequence has not been specified explicitly. In such a case, the default
value for MAXVALUE becomes one of the following:
- For an ascending sequence, the value is the maximum value of the data type that is associated with the sequence.
- For a descending sequence, the value is the START WITH value or -1 if START WITH is not specified.
To find the maximum possible value for a given data type, see Limits in Db2 for z/OS.
- CYCLE or NO CYCLE
whether or not the sequence should continue to generate values after
reaching either its maximum or minimum value. The boundary of the
sequence can be reached either with the next value landing exactly
on the boundary condition or by overshooting it. The default is NO
- Specifies that the sequence continue to generate values after
either the maximum or minimum value has been reached. If this option
is used, after an ascending sequence reaches its maximum value, it
generates its minimum value. After a descending sequence reaches its
minimum value, it generates its maximum value. The maximum and minimum
values for the sequence defined by the MINVALUE and MAXVALUE options
determine the range that is used for cycling.
When CYCLE is in effect, duplicate values can be generated by the sequence. When a sequence is defined with CYCLE, any application conversion tools for converting applications from other vendor platforms to Db2 should also explicitly specify MINVALUE, MAXVALUE, and START WITH values.
- NO CYCLE
- Specifies that the sequence cannot generate more values once the
maximum or minimum value for the sequence has been reached. The NO
CYCLE option (the default) can be altered to CYCLE at any time during
the life of the sequence.
When the next value is being generated for a sequence if the maximum value (for an ascending sequence) or the minimum value (for a descending sequence) of the logical range of the sequence is exceeded and the NO CYCLE option is in effect, an error occurs.
- CACHE or NO CACHE
whether or not to keep some preallocated values in memory for faster
access. This is a performance and tuning option.
- CACHE integer-constant
- Specifies the maximum number of values of the sequence that Db2 can preallocate and keep in
memory. Preallocating values in the cache reduces synchronous I/O
when values are generated for the sequence. The actual number of values
that Db2 caches is always the
lesser of the number in effect for the CACHE option and the number
of remaining values within the logical range. Thus, the CACHE value
is essentially an upper limit for the size of the cache.
In the event the system is shut down (either normally or through a system failure), all cached sequence values that have not been used in committed statements are lost (that is, they will never be used). The value specified for the CACHE option is the maximum number of sequence values that could be lost when the system is shut down.
The minimum value is 2. The default is CACHE 20.
In a data sharing environment, you can use the CACHE and NO ORDER options to allow multiple Db2 members to cache sequence values simultaneously.
- NO CACHE
- Specifies that values of the sequence are not to be preallocated. This option ensures that there is not a loss of values in the case of a system failure. When NO CACHE is specified, the values of the sequence are not stored in the cache. In this case, every request for a new value for the sequence results in synchronous I/O to the log.
- ORDER or NO ORDER
whether the sequence numbers must be generated in order of request.
The default is NO ORDER.
- Specifies that the sequence numbers are generated in order of request. Specifying ORDER might disable the caching of values. There is no guarantee that values are assigned in order across the entire server unless NO CACHE is also specified. ORDER applies only to a single-application process.
- NO ORDER
- Specifies that the sequence numbers do not need to be generated in order of request.
In a data sharing environment, if the CACHE and NO ORDER options are in effect, multiple caches can be active simultaneously, and the requests for next value assignments from different Db2 members might not result in the assignment of values in strict numeric order. For example, if members DB2A and DB2B are using the same sequence, and DB2A gets the cache values 1 to 20 and DB2B gets the cache values 21 to 40, the actual order of values assigned would be 1,21,2 if DB2A requested for next value first, then DB2B requested, and then DB2A again requested. Therefore, to guarantee that sequence numbers are generated in strict numeric order among multiple Db2 members using the same sequence concurrently, specify the ORDER option.
- Owner privileges
- The owner is authorized to change (ALTER privilege) or use (USAGE privilege) the sequence and grant others these privileges. See GRANT (sequence privileges). For more information about ownership of the object see Authorization, privileges, permissions, masks, and object ownership.
- Relationship of MINVALUE and MAXVALUE
- MINVALUE must not be greater than MAXVALUE. Although MINVALUE is typically less than MAXVALUE, MINVALUE can equal MAXVALUE. If START WITH were the same value as MINVALUE and MAXVALUE, the sequence would be constant. The request for the next value in a constant sequence appears to have no effect because all of the values that are generated by the sequence are in fact the same value.
- Defining sequences that cycle
When you define a sequence, you can choose to have it cycle automatically or not when the maximum or minimum value for the sequence has been reached.
- Implicitly or explicitly defining a sequence with NO CYCLE causes the sequence to not cycle automatically after the boundary is reached. However, you can use the ALTER SEQUENCE statement to cycle the sequence manually. ALTER SEQUENCE allows you to restart or extend the sequence, which causes sequence values to continue to be generated.
- Explicitly defining a sequence with CYCLE causes the sequence to cycle automatically after the boundary is reached. Sequence values continue to be generated after the sequence cycles.
When a sequence is defined to cycle automatically, the maximum or minimum value that is generated for a sequence might not be the actual MAXVALUE or MINVALUE value that is specified if the increment is a value other than 1 or -1. For example, the sequence defined with START WITH=1, INCREMENT=2, MAXVALUE=10 will generate a maximum value of 9, and will not generate the value 10.
When a sequence is defined with CYCLE, any application conversion tools (for converting applications from other vendor platforms to Db2) should also explicitly specify MINVALUE, MAXVALUE, and START WITH.
- Defining a constant sequence
You can define a sequence such that it always returns the same (or a constant) value. To create a constant sequence, use either of these techniques when defining the sequence:
- Specify an INCREMENT value of zero and a START WITH value that does not exceed MAXVALUE. Use this option in most cases, and especially if the constant sequence is used often, such as for transaction workloads.
- Specify the same value for START WITH, MINVALUE, and MAXVALUE, and specify CYCLE. This option requires more log write activity and can result in longer wait times, especially if the sequence is used often.
A constant sequence can be used as a numeric global variable. You can use ALTER SEQUENCE to adjust the values that are generated for a constant sequence.
- Consumed values of a sequence
- After Db2 generates a value for a sequence, that value can be said to be "consumed" regardless of whether or not that value is used by the application or not. The value is not reused within the current cycle. A consumed value might not be used when the statement that caused the value to be generated fails for some reason or is rolled back after the value was generated. Generated but unused values can constitute gaps in a sequence.
- Gaps in a sequence
Consecutive values in a sequence differ by the constant INCREMENT BY value specified for the sequence. However, gaps can occur in the values that are assigned to a sequence object by Db2.
The following situations are some examples of how gaps can be introduced in the sequence values:
- A transaction has advanced the sequence and then rolls back.
- The SQL statement leading to the generation of the next value fails after the value was generated.
- The NEXT VALUE expression is used in the SELECT statement of a cursor in a DRDA environment where the client uses block-fetch and not all retrieved rows are fetched by the application.
- The sequence is altered and then the alteration is rolled back.
- The sequence (or an identity column table) is dropped and then the drop is rolled back.
- The SYSIBM.SYSSEQ table space is stopped or closed for any reason (including when DSMAX is reached)
- The Db2 subsystem is stopped or goes down
Values of such gaps are not available for the current cycle, unless the sequence is altered and restarted in a specific way to make them available.
A sequence is incremented independently of a transaction. Thus, a given transaction increments the sequence two times might see a gap in the two numbers that it receives if other transactions concurrently increment the same sequence. Most applications can tolerate these instances as these are not really gaps.
- Duplicate sequence values
- It is possible the duplicate values can be generated for a sequence. Duplicate values are most likely to occur when a sequence is defined with the CYCLE option, is defined as a constant sequence, or is altered. For example, the following situations could cause duplicate sequence values:
- A sequence is defined with the attributes START WITH=2, INCREMENT BY 2, MINVALUE=2, MAXVALUE=10, and CYCLE.
- The ALTER SEQUENCE statement is used to restart the sequence with a value that has already been generated.
- The ALTER SEQUENCE statement is used to reverse the ascending direction of a sequence by changing the INCREMENT BY value from a positive to a negative.
- Using sequences
A sequence can be referenced using a sequence-reference. A sequence reference can appear in most places that an expression can appear. A sequence reference can specify whether the value to be returned is a newly generated value or the previously generated value. A NEXT VALUE sequence expression is used to generate a new value. A PREVIOUS VALUE sequence expression is used to obtain the last assigned value of a sequence. For more information, see Sequence reference.
- Alternative syntax and synonyms
To provide compatibility with previous releases of Db2 or other products in the Db2 family, Db2 supports the following keywords:
- NOMINVALUE (single key word) as a synonym for NO MINVALUE
- NOMAXVALUE (single key word) as a synonym for NO MAXVALUE
- NOCYCLE (single key word) as a synonym for NO CYCLE
- NOCACHE (single key word) as a synonym for NO CACHE
- NOORDER (single key word) as a synonym for NO ORDER
- Example 1
- Create a sequence names "org_seq" that starts at 1 increments by 1, does not cycle, and caches 24 values at a time:
INCREMENT 1, NO MAXVALUE, and NO CYCLE are defaults and do not need to be specified.
CREATE SEQUENCE ORDER_SEQ START WITH 1 INCREMENT BY 1 NO MAXVALUE NO CYCLE CACHE 24;
- Example 2
- The following example shows how to create and use a sequence named "order_seq" in a table named "orders":
or to update the orders:
CREATE SEQUENCE ORDER_SEQ START WITH 1 INCREMENT BY 1 NO MAXVALUE NO CYCLE CACHE 20; INSERT INTO ORDERS (ORDERNO, CUSTNO) VALUES (NEXT VALUE FOR ORDER_SEQ, 123456);
UPDATE ORDERS SET ORDERNO = NEXT VALUE FOR ORDER_SEQ WHERE CUSTNO = 123456;
- Example 3
- The following example shows how to use the same sequence number as a unique key value in two separate tables by referencing the sequence number with a NEXT VALUE expression for the first row to generate the sequence value and with a PREVIOUS VALUE expression for the other rows to refer to the sequence value most recently generated.
INSERT INTO ORDERS (ORDERNO, CUSTNO) VALUES (NEXT VALUE FOR ORDER_SEQ, 123456); INSERT INTO LINE_ITEMS (ORDERNO, PARTNO, QUANTITY) VALUES (PREVIOUS VALUE FOR ORDER_SEQ, 987654, 100);
If NEXT VALUE is invoked in the same statement as the PREVIOUS VALUE, then regardless of their order in the statement, PREVIOUS VALUE returns the previous (unincremented) value and NEXT VALUE returns the next value.