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    Wednesday, 18 December 2019

    Sequences in SQL


    Sequences
    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 .
    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.

    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 .
    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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