数据库中序列（sequence）的用法

CREATE SEQUENCE statement

The CREATE SEQUENCE statement defines a sequence at the application server.

Invocation

This statement can be embedded in an application program or issued through the use of dynamic SQL statements. It is an executable statement that can be dynamically prepared only if DYNAMICRULES run behavior is in effect for the package (SQLSTATE 42509).

Authorization

The privileges held by the authorization ID of the statement must include at least one of the following:

22 IMPLICIT_SCHEMA privilege on the database, if the implicit or 2explicit schema name of the sequence does not exist2CREATEIN privilege on the schema, if the schema name of the 2sequence refers to an existing schema2SYSADM or DBADM authority

Syntax

                                      .-AS INTEGER----.
>>-CREATE SEQUENCE--sequence-name--*--+---------------+--*------>
                                      '-AS--data-type-'

>--+------------------------------+--*-------------------------->
   '-START WITH--numeric-constant-'

   .-INCREMENT BY 1-----------------.
>--+--------------------------------+--*------------------------>
   '-INCREMENT BY--numeric-constant-'

   .-NO MINVALUE----------------.
>--+----------------------------+--*---------------------------->
   '-MINVALUE--numeric-constant-'

   .-NO MAXVALUE----------------.     .-NO CYCLE-.
>--+----------------------------+--*--+----------+--*----------->
   '-MAXVALUE--numeric-constant-'     '-CYCLE----'

   .-CACHE 20----------------.     .-NO ORDER-.
>--+-------------------------+--*--+----------+--*-------------><
   +-CACHE--integer-constant-+     '-ORDER----'
   '-NO CACHE----------------'

Description

sequence-name

Names the sequence. The combination of name, and the implicit or explicit schema name must not identify an existing sequence at the current server (SQLSTATE 42710).

The unqualified form of sequence-name is an SQL identifier. The qualified form is a qualifier followed by a period and an SQL identifier. The qualifier is a schema name.

If the sequence name is explicitly qualified with a schema name, the schema name cannot begin with 'SYS' or an error (SQLSTATE 42939) is raised.

AS data-type

Specifies the data type to be used for the sequence value. The data type can be any exact numeric type (SMALLINT, INTEGER, BIGINT or DECIMAL) with a scale of zero, or a user-defined distinct type or reference type for which the source type is an exact numeric type with a scale of zero (SQLSTATE 42815). The default is INTEGER.

START WITH numeric-constant

Specifies the first value for the sequence. This value can be any positive or negative value that could be assigned to a column of the data type associated with the sequence (SQLSTATE 42815), without non-zero digits existing to the right of the decimal point (SQLSTATE 428FA). The default is 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

Specifies the interval between consecutive values of the sequence. This value can be any positive or negative value that could be assigned to a column of the data type associated with the sequence (SQLSTATE 42815), and does not exceed the value of a large integer constant (SQLSTATE 42820), without non-zero digits existing to the right of the decimal point (SQLSTATE 428FA).

If this value is negative, this is a descending sequence. If this value is 0 or positive, this is an ascending sequence. The default is 1.

MINVALUE or NO MINVALUE

Specifies 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.

MINVALUE numeric-constant

Specifies the numeric constant that is the minimum value. This value can be any positive or negative value that could be assigned to a column of the data type associated with the sequence (SQLSTATE 42815), without non-zero digits existing to the right of the decimal point (SQLSTATE 428FA), but the value must be less than or equal to the maximum value (SQLSTATE 42815).

NO MINVALUE

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 associated with the sequence. This is the default.

MAXVALUE or NO MAXVALUE

Specifies the maximum value at which an ascending sequence either cycles or stops generating values, or a descending sequence cycles to after reaching the minimum value.

MAXVALUE numeric-constant

Specifies the numeric constant that is the maximum value. This value can be any positive or negative value that could be assigned to a column of the data type associated with the sequence (SQLSTATE 42815), without non-zero digits existing to the right of the decimal point (SQLSTATE 428FA), but the value must be greater than or equal to the minimum value (SQLSTATE 42815).

NO MAXVALUE

For an ascending sequence, the value is the maximum value of the data type associated with the sequence. For a descending sequence, the value is the START WITH value, or -1 if START WITH is not specified.

CYCLE or NO CYCLE

Specifies whether 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.

CYCLE

Specifies that values continue to be generated for this sequence after 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 determine the range that is used for cycling.

When CYCLE is in effect, then duplicate values can be generated for the sequence.

NO CYCLE

Specifies that values will not be generated for the sequence once the maximum or minimum value for the sequence has been reached. This is the default.

CACHE or NO CACHE

Specifies whether 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 sequence values that are preallocated and kept in memory. Preallocating and storing values in the cache reduces synchronous I/O to the log when values are generated for the sequence.

In the event of 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 in case of system failure.

The minimum value is 2 (SQLSTATE 42815). The default value is CACHE 20.

NO CACHE

Specifies that values of the sequence are not to be preallocated. It ensures that there is not a loss of values in the case of a system failure, shutdown or database deactivation. When this option 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.

NO ORDER or ORDER

Specifies whether the sequence numbers must be generated in order of request.

ORDER

Specifies that the sequence numbers are generated in order of request.

NO ORDER

Specifies that the sequence numbers do not need to be generated in order of request. This is the default.

Notes

• 1It is possible to define a constant sequence, that is, 1one that would always return a constant value. 1This could be done by specifying an INCREMENT value of zero and a 1START WITH value that does not exceed MAXVALUE, or by specifying the 1same value for START WITH, MINVALUE and MAXVALUE. 1For a constant sequence, each time NEXT VALUE is invoked for the1sequence, the same value is returned. 1A constant sequence can be used as a numeric global variable. 1ALTER SEQUENCE can be used to adjust the values that will be 1generated for a constant sequence.
• A sequence can be cycled manually by using the ALTER SEQUENCE statement. If NO CYCLE is implicitly or explicitly specified, the sequence can be restarted or extended using the ALTER SEQUENCE statement to cause values to continue to be generated once the maximum or minimum value for the sequence has been reached.
• A sequence can be explicitly defined to cycle by specifying the CYCLE keyword. Use the CYCLE option when defining a sequence to indicate that the generated values should cycle once the boundary is reached. When a sequence is defined to automatically cycle (that is, CYCLE was explicitly specified), the maximum or minimum value generated for a sequence might not be the actual MAXVALUE or MINVALUE 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 defining a sequence with CYCLE, carefully consider the impact of the values for MINVALUE, MAXVALUE and START WITH.
• Caching sequence numbers implies that a range of sequence numbers can be kept in memory for fast access. When an application accesses a sequence that can allocate the next sequence number from the cache, the sequence number allocation can happen quickly. However, if an application accesses a sequence that cannot allocate the next sequence number from the cache, the sequence number allocation may require having to wait for I/O operations to persistent storage. The choice of the value for CACHE should be done keeping in mind the performance and application requirements tradeoffs.
• The definer of a sequences is granted ALTER and USAGE privileges with the grant option. The definer can also drop the sequence.
• Compatibilities
  • For compatibility with previous versions of DB2:
    • A comma can be used to separate multiple sequence options
  • The following syntax is also supported:
    • NOMINVALUE, NOMAXVALUE, NOCYCLE, NOCACHE, and NOORDER.

Examples

Example 1: Create a sequence called ORG_SEQ that starts at 1, increments by 1, does not cycle, and caches 24 values at a time:

   CREATE SEQUENCE ORG_SEQ
     START WITH 1
     INCREMENT BY 1
     NO MAXVALUE
     NO CYCLE
     CACHE 24