SQL> DROP TABLE CHILD PURGE
  2  /

Table dropped.

SQL> DROP TABLE PARENT PURGE
  2  /

Table dropped.

SQL> CREATE TABLE PARENT(
  2                      ID NUMBER
  3                     )
  4  /

Table created.

SQL> ALTER TABLE PARENT
  2    ADD CONSTRAINT PK_PARENT
  3      PRIMARY KEY(
  4                  ID
  5                 )
  6  /

Table altered.

SQL> CREATE TABLE CHILD(
  2                     ID NUMBER,
  3                     PARENT_ID NUMBER
  4                    )
  5  /

Table created.

SQL> ALTER TABLE CHILD
  2    ADD CONSTRAINT PK_CHILD
  3      PRIMARY KEY(
  4                  ID
  5                 )
  6  /

Table altered.

SQL> INSERT
  2    INTO CHILD
  3    VALUES(
  4           1,
  5           1
  6          )
  7  /

1 row created.

SQL> ALTER TABLE CHILD
  2    ADD CONSTRAINT FK_CHILD
  3      FOREIGN KEY(
  4                  PARENT_ID
  5                 )
  6      REFERENCES PARENT
  7      ENABLE NOVALIDATE
  8  /

Table altered.

SQL> UPDATE CHILD
  2     SET ID = 2
  3  /

1 row updated.

SQL> UPDATE CHILD
  2     SET PARENT_ID = PARENT_ID
  3  /
UPDATE CHILD
*
ERROR at line 1:
ORA-02291: integrity constraint (SCOTT.FK_CHILD) violated - parent key not
found


SQL>