ORACLE 12C Foundation
1. CONSTRAID ID 0 is cdb,1 is cdb$root, 2 is pdb seed,3 and above is pdb
two。 Self-increasing sequence
Before 12c, Oracle could only implement this function through sequence.
Sys@newtestCDB > create table test (id number generated always as identity, name varchar2 (20))
Table created.
Elapsed: 00:00:00.04
Sys@newtestCDB > insert into test (name) values ('smith')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > insert into test (name) values ('smith3')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > insert into test (name) values ('smith4')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > select * from test
ID NAME 1 smith 2 smith3 3 smith4
Elapsed: 00:00:00.02
Sys@newtestCDB > update test set id=1 where id=2
Update test set id=1 where id=2
*
ERROR at line 1:
ORA-32796: cannot update a generated always identity column
Elapsed: 00:00:00.03
Sys@newtestCDB > insert into test (id,name) values (null,'smith4')
Insert into test (id,name) values (null,'smith4')
*
ERROR at line 1:
ORA-32795: cannot insert into a generated always identity column
Elapsed: 00:00:00.01
Sys@newtestCDB > insert into test (id,name) values (2remith4')
Insert into test (id,name) values (2MIT 4')
*
ERROR at line 1:
ORA-32795: cannot insert into a generated always identity column
Sys@newtestCDB > delete from test where id=3
1 row deleted.
Elapsed: 00:00:00.03
Sys@newtestCDB > insert into test (name) values ('smith5')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > select * from test
ID NAME 2 smith3 4 smith5
Elapsed: 00:00:00.01
Conclusion:
GENERATED ALWAYS AS IDENTITY can insert without specifying this column
GENERATED ALWAYS AS IDENTITY cannot insert a null value in this column
GENERATED ALWAYS AS IDENTITY cannot specify specific values to insert
GENERATED ALWAYS AS IDENTITY cannot update this column with update
Sys@newtestCDB > create table test (id number generated by default as identity, name varchar2 (20))
Table created.
Elapsed: 00:00:00.03
Sys@newtestCDB > insert into test (name) values ('smith')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > insert into test (name) values ('smith3')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > insert into test (name) values ('smith4')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > insert into test (id,name) values (null,'smith4')
Insert into test (id,name) values (null,'smith4')
*
ERROR at line 1:
ORA-01400: cannot insert NULL into ("SYS". "TEST". "ID")
Elapsed: 00:00:00.02
Sys@newtestCDB > insert into test (id,name) values (2remith3')
1 row created.
Elapsed: 00:00:00.01
Sys@newtestCDB > update test set id= NULL where id=2
Update test set id= NULL where id=2
*
ERROR at line 1:
ORA-01407: cannot update ("SYS". "TEST". "ID") to NULL
Conclusion:
GENERATED BY DEFAULT AS IDENTITY can insert without specifying this column
GENERATED BY DEFAULT AS IDENTITY cannot insert a null value in this column
GENERATED BY DEFAULT AS IDENTITY can specify specific values to insert
GENERATED BY DEFAULT AS IDENTITY can update the column using update, but not to NULL
Sys@newtestCDB > create table test (id number generated by default ON NULL as identity, name varchar2 (20))
Table created.
Elapsed: 00:00:00.03
Sys@newtestCDB > insert into test (id,name) values (null,'smith4')
1 row created.
Elapsed: 00:00:00.01
Conclusion:
GENERATED BY DEFAULT ON NULL AS IDENTITY can insert without specifying this column
In GENERATED BY DEFAULT ON NULL AS IDENTITY mode, you can specify specific values to insert.
GENERATED BY DEFAULT ON NULL AS IDENTITY can insert null values in this column
GENERATED BY DEFAULT ON NULL AS IDENTITY can update this column using update
Sys@newtestCDB > col tablename format A20
Sys@newtestCDB > col table_name format A20
Sys@newtestCDB > col sequence_name format A20
Sys@newtestCDB > SELECT a.name AS table_name
2 b.name AS sequence_name
3 FROM sys.idnseq$ c
4 JOIN obj$ an ON c.obj# = a.obj#
5 JOIN obj$ b ON c.seqobj# = b.obj#
6 where a.nameplate tests
TABLE_NAME SEQUENCE_NAME
TEST ISEQ$$_83962
Elapsed: 00:00:00.01
Sys@newtestCDB > create table test (id number generated by default as identity, name varchar2 (20))
Table created.
Elapsed: 00:00:00.03
Sys@newtestCDB > SELECT a.name AS table_name
2 b.name AS sequence_name
3 FROM sys.idnseq$ c
4 JOIN obj$ an ON c.obj# = a.obj#
5 JOIN obj$ b ON c.seqobj# = b.obj#
6 where a.nameplate tests
TABLE_NAME SEQUENCE_NAME
TEST ISEQ$$_83964
Elapsed: 00:00:00.01
Sys@newtestCDB > SELECT object_name, object_type FROM user_objects where object_name='ISEQ$$_83964'
OBJECT_NAMEOBJECT_TYPE
ISEQ$$_83964
SEQUENCE
Elapsed: 00:00:00.05
Sys@newtestCDB > drop table test
Table dropped.
Elapsed: 00:00:00.04
Sys@newtestCDB > SELECT object_name, object_type FROM user_objects where object_name='ISEQ$$_83964'
No rows selected
Elapsed: 00:00:00.00
Sys@newtestCDB > create table test (id number generated by default as identity, name varchar2 (20))
Table created.
Elapsed: 00:00:00.06
Sys@newtestCDB > SELECT a.name AS table_name
2 b.name AS sequence_name
3 FROM sys.idnseq$ c
4 JOIN obj$ an ON c.obj# = a.obj#
5 JOIN obj$ b ON c.seqobj# = b.obj#
6 where a.nameplate tests
TABLE_NAME SEQUENCE_NAME
TEST ISEQ$$_83966
Elapsed: 00:00:00.01
Sys@newtestCDB > drop SEQUENCE ISEQ$$_83966
Drop SEQUENCE ISEQ$$_83966
*
ERROR at line 1:
ORA-32794: cannot drop a system-generated sequence
Elapsed: 00:00:00.02
Conclusion:
Identity Columns is implemented based on sequence.
Sequence in GENERATED IDENTITY cannot be deleted alone
Delete the sequence table in GENERATED IDENTITY when it is deleted
This is a picture of 12.1, and there are new changes in 12.2.
