在12c之前，如果想实现column的自动增长，通常是通过sequence来实现，然而这个问题在12c中得到了解决，Oracle推迟了identity column功能，该功能完美的解决了之前的问题，下面是我的简单测试，供参考！
SQL> alter pluggable database all open;
Pluggable database altered.
SQL> select name,open_mode from V$pdbs;
NAME OPEN_MODE
------------------------------ ----------
PDB$SEED READ ONLY
PDBORCL READ WRITE
SQL> conn roger/roger@pdborcl
Connected.
SQL> show con_name
CON_NAME
------------------------------
PDBORCL
SQL> create table test(id number generated by default as identity ,
2 name varchar2(20));
Table created.
SQL> insert into test(name) values('roger');
1 row created.
SQL> insert into test(name) values('killdb.com');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from test;
ID NAME
---------- --------------------
1 roger
2 killdb.com
SQL>
SQL> insert into test(id,name) values(null,'killdb.com');
insert into test(id,name) values(null,'killdb.com')
*
ERROR at line 1:
ORA-01400: cannot insert NULL into ("ROGER"."TEST"."ID")
可以看到id列默认可以进行自动增长，在12c之前，Oracle只能通过sequence来实现这个功能。 另外我们还可以看到，这种情况下，是不能插入null值的。
SQL> alter table test modify (id default null);
alter table test modify (id default null)
*
ERROR at line 1:
ORA-30674: identity column cannot have a default value
那么对于identity 的column，真的不能插入null值吗？ 其实是可以的，不过你得这样做：
SQL> create table test1 (id number generated by default on NULL as identity ,name varchar2(20));
Table created.
SQL>
SQL> insert into test1 values(1,'killdb.com');
1 row created.
SQL> insert into test1 values(2,'baidu.com');
1 row created.
SQL> insert into test1 values(null,'google.com');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from test1;
ID NAME
---------- --------------------
1 killdb.com
2 baidu.com
1 google.com
不过比较奇怪的是，大家看到了，插入的第3条数据的id列为null的情况下，oracle自己实际的值为1. 这个1是怎么来的呢 ？后面会告诉你答案。
SQL> update test1 set id=100 where id=2;
1 row updated.
SQL> commit;
Commit complete.
SQL> select * from test1;
ID NAME
---------- --------------------
1 killdb.com
100 baidu.com
1 google.com
这种情况下，可以进行正常的dml，因为identity column默认是为null的，下面继续一个测试。
SQL> create table test2 (id number generated always as identity ,name varchar2(20));
Table created.
SQL> insert into test2(name) values('killdb.com');
1 row created.
SQL> insert into test2(name) values('baidu.com');
1 row created.
SQL> insert into test2(name) values('google.com');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from test2;
ID NAME
---------- --------------------
1 killdb.com
2 baidu.com
3 google.com
SQL> update test2 set id=4 where id=2;
update test2 set id=4 where id=2
*
ERROR at line 1:
ORA-32796: cannot update a generated always identity column
SQL> update test2 set id=1 where id=2;
update test2 set id=1 where id=2
*
ERROR at line 1:
ORA-32796: cannot update a generated always identity column
SQL> delete from test2 where id=2;
1 row deleted.
SQL> commit;
Commit complete.
SQL> select * from test2;
ID NAME
---------- --------------------
1 killdb.com
3 google.com
我们可以看到，居然不能进行update，这里想想也应该知道，Oracle是不会允许你进行update的。
SQL>
SQL> select TABLE_NAME,
2 COLUMN_NAME,
3 DEFAULT_ON_NULL,
4 IDENTITY_COLUMN,
5 DATA_DEFAULT
6 from user_tab_columns
7 where IDENTITY_COLUMN='YES';
TABLE_NAME COLUMN_NAME DEF IDE DATA_DEFAULT
-------------------- -------------------- --- --- ------------------------------------------------------------
TEST ID NO YES "ROGER"."ISEQ$$_91820".nextval
TEST1 ID YES YES "ROGER"."ISEQ$$_91822".nextval
TEST2 ID NO YES "ROGER"."ISEQ$$_91824".nextval
SQL>
SQL> set pagesize 200 long 9999
SQL> select dbms_metadata.get_ddl('TABLE','TEST') from dual;
DBMS_METADATA.GET_DDL('TABLE','TEST')
--------------------------------------------------------------------------------
CREATE TABLE "ROGER"."TEST"
( "ID" NUMBER GENERATED BY DEFAULT AS IDENTITY MINVALUE 1 MAXVALUE 9999999999
999999999999999999 INCREMENT BY 1 START WITH 1 CACHE 20 NOORDER NOCYCLE NOT NULL ENABLE,
"NAME" VARCHAR2(20)
) SEGMENT CREATION IMMEDIATE
PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255
NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "USERS"
SQL> select dbms_metadata.get_ddl('TABLE','TEST1') from dual;
DBMS_METADATA.GET_DDL('TABLE','TEST1')
--------------------------------------------------------------------------------
CREATE TABLE "ROGER"."TEST1"
( "ID" NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY MINVALUE 1 MAXVALUE 99
99999999999999999999999999 INCREMENT BY 1 START WITH 1 CACHE 20 NOORDER NOCYCLE
NOT NULL ENABLE,
"NAME" VARCHAR2(20)
) SEGMENT CREATION IMMEDIATE
PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255
NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "USERS"
最后通过表的定义，我们可以看到，ID列默认被定义为了sequence，从属性来看跟我们之前版本中创建sequence的属性差不多。而且我们看到默认情况下start with 为1。这个也就是为什么前面插入null的情况下id=1的原因。