Sequences
A Sequence is simply an automatic counter, which
generates sequential numbers whenever required. A
sequence can be defined to:
- Generate
numbers in ascending or descending order.
- Provide
intervals between numbers.
- Caching
of sequence numbers in memory to speed up their availability .
A sequence is an independent
object and can be used with any table that requires its output.
Creating a Sequences :
The minimum information required for generating
numbers using a sequence is:
- The
starting number.
- The
maximum number that can be generated by a sequence.
- The
increment value for generating the next number.
Syntax:
CREATE
SEQUENCE <Sequence Name>
[INCREMENT
BY <IntegerValue>
START WITH <Integer Value>
MAXVALUE <Integer Value>
MINVALUE <Integer Value> CYCLE CACHE ]
Example:
create sequence seq increment by 1 start
with 1 maxvalue 999 CYCLE;
INCREMENT BY:
Specifies the interval between sequence numbers. It can be any positive or negative value but not zero. If this clause is omitted, the default value is 1.
MAXVALUE And MINVALUE :
Specifies the maximum or minmum value that a sequence can generate.
START WITH:
Specifies the first sequence number to be generated. The default for an ascending sequence is the sequence minimum value (1) and for a descending sequence, it is the maximum value (-1).
CYCLE:
Specifies that the sequence continues to generate repeat values after reaching either its maximum value or minimum value.
CACHE:
Specifies how many values to generate in advance and to keep in memory for faster access. Minimum value is two for this option.
Specifies the interval between sequence numbers. It can be any positive or negative value but not zero. If this clause is omitted, the default value is 1.
MAXVALUE And MINVALUE :
Specifies the maximum or minmum value that a sequence can generate.
START WITH:
Specifies the first sequence number to be generated. The default for an ascending sequence is the sequence minimum value (1) and for a descending sequence, it is the maximum value (-1).
CYCLE:
Specifies that the sequence continues to generate repeat values after reaching either its maximum value or minimum value.
CACHE:
Specifies how many values to generate in advance and to keep in memory for faster access. Minimum value is two for this option.
Altering a Sequence :
A sequence once created can be altered.
This is achieved by using the ALTER SEQUENCE statement.
The START value of the sequence cannot be altered .
The START value of the sequence cannot be altered .
Syntax:
ALTER SEQUENCE <SequenceName> [INCREMENT BY <IntegerValue>
MINVALUE <IntegerValue> ] ;
Example:
Alter sequence seq increment by 2;
Destroying a Sequence:
Syntax:
DROP SEQUENCE sequencename;
Example:
drop sequence seq;
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