Oracle 12c中identity column學習筆記

在12c之前，如果想實現column的自動成長，通常是通過sequence來實現，然而這個問題在12c中得到瞭解決，Oracle延遲了identity column功能，該功能完美的解決了之前的問題，下面是我的簡單測試，供參考！

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('111cn.net');
 
1 row created.
 
SQL> commit;
 
Commit complete.
 
SQL> select * from test;
 
        ID NAME
---------- --------------------
         1 roger
         2 111cn.net
 
SQL>
SQL> insert into test(id,name) values(null,'111cn.net');
insert into test(id,name) values(null,'111cn.net')
                                 *
ERROR at line 1:
ORA-01400: cannot insert NULL into ("ROGER"."TEST"."ID")

可以看到id列預設可以進行自動成長，在12c之前，Oracle只能通過sequence來實現這個功能。 另外我們還可以看到，這種情況下，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

那麼對於identity 的column，真的不能插入null值嗎？ 其實是可以的，不過你得這樣做：

<pre class="brush:php">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,'111cn.net');
 
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 111cn.net
         2 baidu.com
         1 google.com</pre>

不過比較奇怪的是，大家看到了，插入的第3條資料的id列為null的情況下，oracle自己實際的值為1. 這個1是怎麼來的呢 ？後面會告訴你答案。

SQL> update test1 set id=100 where id=2;
 
1 row updated.
 
SQL> commit;
 
Commit complete.
 
SQL> select * from test1;
 
        ID NAME
---------- --------------------
         1 111cn.net
       100 baidu.com
         1 google.com

這種情況下，可以進行正常的dml，因為identity column預設是為null的，下面繼續一個測試。

SQL> create table test2 (id number generated always as identity  ,name varchar2(20));
 
Table created.
 
SQL> insert into test2(name) values('111cn.net');
 
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 111cn.net
         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 111cn.net
         3 google.com

我們可以看到，居然不能進行update，這裡想想也應該知道，Oracle是不會允許你進行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"
最後通過表的定義，我們可以看到，ID列預設被定義為了sequence，從屬性來看跟我們之前版本中建立sequence的屬性差不多。而且我們看到預設情況下start with 為1。這個也就是為什麼前面插入null的情況下id=1的原因。