106. SYNONYM AND SEQUENCE

SYNONYM

A synonym is a database object, which is used as an alias for a table, view or sequence.

TYPES

Ø  Private

Ø  Public

Private synonym is available to the particular user who creates.

Public synonym is created by DBA which is available to all the users.

ADVANTAGES

Ø  Hide the name and owner of the object.

Ø  Provides location transparency for remote objects of a distributed database.

CREATE AND DROP

SQL> create synonym s1 for emp;

SQL> create public synonym s2 for emp;

SQL> drop synonym s1;

SEQUENCE

A sequence is a database object, which can generate unique, sequential integer values.

It can be used to automatically generate primary key or unique key values.

A sequence can be either in an ascending or descending order.

Syntax:

      Create sequence <seq_name> [increment bty n] [start with n] [maxvalue n]

                                  [minvalue n] [cycle/nocycle] [cache/nocache];

By defalult the sequence starts with 1, increments by 1 with minvalue of 1 and with nocycle,  nocache.

Cache option pre-alloocates a set of sequence numbers and retain them in memory for faster access.

Ex:

     SQL> create sequence s;

     SQL> create sequence s increment by 10 start with 100 minvalue 5 maxvalue 200 cycle  

             cache 20;

USING SEQUENCE

SQL> create table student(no number(2),name varchar(10));

SQL> insert into student values(s.nextval, ‘saketh’);

Ø  Initially currval is not defined and nextval is starting value.

Ø  After that nextval and currval are always equal.

CREATING ALPHA-NUMERIC SEQUENCE

SQL> create sequence s start with 111234;

SQL> Insert into student values (s.nextval || translate  

         (s.nextval,’1234567890’,’abcdefghij’));

ALTERING SEQUENCE

We can alter the sequence to perform the following.

Ø  Set or eliminate minvalue or maxvalue.

Ø  Change the increment value.

Ø  Change the number of cached sequence numbers.

Ex:

     SQL> alter sequence s minvalue 5;

     SQL> alter sequence s increment by 2;

     SQL> alter sequence s cache 10;

DROPPING SEQUENCE

SQL> drop sequence s;