Why primaru key instace is null

Question

when i send parameters values of
create method (create("suman","kumar")) through client file, it inserts data correctly in table
but it gives following error(Table stru:- (id int auto_increment primary key, fname varchar(20), lname varchar(20)):-
javax.ejb.CreateException: Primary key for created instance is null.
at org.jboss.ejb.plugins.cmp.jdbc.JDBCStoreManager.createEntity(JDBCStoreManager.java:520)
at org.jboss.ejb.plugins.CMPPersistenceManager.createEntity(CMPPersistenceManager.java:208)
at org.jboss.resource.connectionmanager.CachedConnectionInterceptor.createEntity(CachedConnectionInterceptor.java:269)
at org.jboss.ejb.EntityContainer.createHome(EntityContainer.java:736)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(

Answer 1

Now if primary key does not allow NULL value in column than why it is allow to create primary key on newly created table?
The table does not have any rows, so there is no NULL value.
If you would have inserted a NULL value first, and try to create the primary key afterwards, the primary key would not be created.
sql> create table test1 (id number)
  2  /
Table created.
sql> alter table test1 add constraint test1_pk primary key (id)
  2  /
Table altered.
sql> insert into test1(id) values (null)
  2  /
insert into test1(id) values (null)
ERROR at line 1:
ORA-01400: cannot insert NULL into ("P1EMDA"."TEST1"."ID")
sql> alter table test1 drop primary key
  2  /
Table altered.
sql> insert into test1(id) values (null)
  2  /
1 row created.
sql> alter table test1 add constraint test1_pk primary key (id)
  2  /
alter table test1 add constraint test1_pk primary key (id)
ERROR at line 1:
ORA-01449: column contains NULL values; cannot alter to NOT NULL
sql> drop table test1
  2  /
Table dropped.