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generated as identity tips

Oracle Database Tips by Donald BurlesonJuly 4, 2015

 

Question:  What is "generated as identity" feature in 12c, and how does "generated as identify" compare to the concept of an Oracle sequence?

 

 
Answer:  In all relational databases, finding a unique key to identify each row has always been problematic.  Oracle first introduced "sequences ", or linear number generators" wo allow for the specification of unique keys for tables and this has worked just fine.
 
With traditional sequences, the DBA has to define the sequence and the developer had to use the "nextval" operator in order ro insert into the table:
 
Now, starting with Oracle 12c, we can use the another method, generated as identity to ensure that each row for a table is unique.  The "generated as identify" syntax is very simplar ti the Java and DB2 equivalent.

 

SEQUENCE GENERATED AS IDENTITY
create sequence
   customer_seq
increment by 1
start with 1;
 
No equivalent needed


create table
  customer
 (cust_id number,
  test_name varchar2(20));

create table
   customer
  (cust_id number
      generated as identity ,
   cust_name varchar2(20));



Insert into
   customer
values
   (customer_seq.nextval, 'XXX');
Insert into
   customer
values
   ('XXX');

 

From the example insert above, note that there is not need to reference the "identity" column" you simply omit any reference to it and Oracle will automatically increment the key and insert it into the table on your behalf.


If you try to enter a value for a generated as identity column you will get an ORA-32795    error:

insert into customer values (1,'XXX');
insert into test values (1,'XXX')
*
ERROR at line 1:
ORA-32795: cannot insert into a generated always identity column

 

   
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