DROP TABLE t PURGE;

CREATE TABLE t (
  ID NUMBER GENERATED ALWAYS AS IDENTITY,
  text VARCHAR2(50),
  CONSTRAINT id_pk PRIMARY KEY (ID)
);

SQL> CREATE TABLE t (
  2    ID NUMBER GENERATED ALWAYS AS IDENTITY,
  3    text VARCHAR2(50),
  4    CONSTRAINT id_pk PRIMARY KEY (ID)
  5  );

Table created.

SQL>
SQL> INSERT ALL
  2    INTO t (text) VALUES ('a')
  3    INTO t (text) VALUES ('b')
  4    INTO t (text) VALUES ('c')
  5    INTO t (text) VALUES ('d')
  6  SELECT * FROM dual;
INSERT ALL
*
ERROR at line 1:
ORA-00001: unique constraint (LALIT.ID_PK) violated


SQL>

SQL> INSERT INTO t(text) SELECT 'Level '||LEVEL FROM dual CONNECT BY LEVEL <= 4;

4 rows created.

SQL>
SQL> SELECT * FROM t;

        ID TEXT
---------- --------------------------------------------------
         2 Level 1
         3 Level 2
         4 Level 3
         5 Level 4

SQL>

SQL> ALTER TABLE t
  2    DISABLE CONSTRAINT id_pk;

Table altered.

SQL>
SQL> INSERT ALL
  2    INTO t (text) VALUES ('a')
  3    INTO t (text) VALUES ('b')
  4    INTO t (text) VALUES ('c')
  5    INTO t (text) VALUES ('d')
  6  SELECT * FROM dual;

4 rows created.

SQL>
SQL> SELECT * FROM t;

        ID TEXT
---------- ----------------------
         2 Level 1
         3 Level 2
         4 Level 3
         5 Level 4
         6 a
         6 b
         6 c
         6 d

8 rows selected.

SQL>

SQL> DROP SEQUENCE s;

Sequence dropped.

SQL>
SQL> CREATE SEQUENCE s;

Sequence created.

SQL>
SQL> DROP TABLE t PURGE;

Table dropped.

SQL>
SQL> CREATE TABLE t (
  2    ID NUMBER,
  3    text VARCHAR2(50),
  4    CONSTRAINT id_pk PRIMARY KEY (ID)
  5  );

Table created.

SQL>
SQL> INSERT ALL
  2    INTO t VALUES (s.nextval, 'a')
  3    INTO t VALUES (s.nextval, 'b')
  4    INTO t VALUES (s.nextval, 'c')
  5    INTO t VALUES (s.nextval, 'd')
  6  SELECT * FROM dual;
INSERT ALL
*
ERROR at line 1:
ORA-00001: unique constraint (LALIT.ID_PK) violated


SQL>
SQL> SELECT * FROM T;

no rows selected

SQL>
SQL> ALTER TABLE t
  2    DISABLE CONSTRAINT id_pk;

Table altered.

SQL> INSERT ALL
  2    INTO t VALUES (s.nextval, 'a')
  3    INTO t VALUES (s.nextval, 'b')
  4    INTO t VALUES (s.nextval, 'c')
  5    INTO t VALUES (s.nextval, 'd')
  6  SELECT * FROM dual;

4 rows created.

SQL> SELECT * FROM T;

        ID TEXT
---------- ----------------------------------------
         2 a
         2 b
         2 c
         2 d

SQL>

SQL> CREATE OR REPLACE TRIGGER t_trg
  2      BEFORE INSERT ON t
  3      FOR EACH ROW
  4      WHEN (new.id IS NULL)
  5      BEGIN
  6        SELECT s.NEXTVAL
  7        INTO   :new.id
  8        FROM   dual;
  9      END;
 10  /

Trigger created.

SQL> truncate table t;

Table truncated.

SQL> INSERT ALL
  2    INTO t (text) VALUES ('a')
  3    INTO t (text) VALUES ('b')
  4    INTO t (text) VALUES ('c')
  5    INTO t (text) VALUES ('d')
  6  SELECT * FROM dual;

4 rows created.

SQL> SELECT * FROM t;

        ID TEXT
---------- -------------------------
         3 a
         4 b
         5 c
         6 d

SQL>