SQL> SQL> create table a(id number,id1 number);
SP2-0734: unknown command beginning "SQL> creat..." - rest of line ignored.
SQL>
SQL> Table created.
SP2-0734: unknown command beginning "Table crea..." - rest of line ignored.
SQL>
SQL> Elapsed: 00:00:00.31
SP2-0734: unknown command beginning "Elapsed: 0..." - rest of line ignored.
SQL> SQL> create table b(id number);
SP2-0734: unknown command beginning "SQL> creat..." - rest of line ignored.
SP2-0044: For a list of known commands enter HELP
and to leave enter EXIT.

SET DEFINE OFF;
CREATE OR REPLACE TRIGGER emp_trig 
BEFORE INSERT ON emp
FOR EACH ROW

DECLARE
pragma autonomous_transaction;	
v_deptno NUMBER:=50;
BEGIN
IF(:NEW.deptno=v_deptno ) THEN
INSERT INTO dept(DEPTNO, DNAME, LOC)VALUES (50,'DEFENCE','INDIANA');
COMMIT;
RAISE_APPLICATION_ERROR(-20000,'Match found can not be inserted');
END IF;

end;


--try with what is matching record
INSERT INTO emp(EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
VALUES(7800,'SAHEED','MANAGER',7839,SYSDATE,3000,NULL,50);

--try with what is not matching
INSERT INTO emp(EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
VALUES(7800,'SAHEED','MANAGER',7839,SYSDATE,3000,NULL,60);


SELECT * FROM dept;
SELECT * FROM emp;

USER                           SYSDATE              NAME      INSTANCE_NUMBER INSTANCE_NAME    HOST_NAME                      VERSION
------------------------------ -------------------- --------- --------------- ---------------- ------------------------------ -------------
KEVIN                          08-mar-2015 11:11:12 ORCL                    1 orcl             FORESE                         11.2.0.1.0

1 row selected.

Elapsed: 00:00:00.01
11:11:12 SQL> drop table a;

Table dropped.

Elapsed: 00:00:00.07
11:11:17 SQL>
11:11:17 SQL> create table a (aid integer not null);

Table created.

Elapsed: 00:00:00.01
11:11:17 SQL>
11:11:17 SQL> create or replace trigger air_a
11:11:17   2  after insert on a
11:11:17   3  for each row
11:11:17   4  begin
11:11:17   5     delete from a where aid = :new.aid;
11:11:17   6  end;
11:11:17   7  /

Trigger created.

Elapsed: 00:00:00.01
11:11:17 SQL> show errors;
No errors.
11:11:17 SQL>
11:11:17 SQL> insert into a values (1);
insert into a values (1)
            *
ERROR at line 1:
ORA-04091: table KEVIN.A is mutating, trigger/function may not see it
ORA-06512: at "KEVIN.AIR_A", line 2
ORA-04088: error during execution of trigger 'KEVIN.AIR_A'


Elapsed: 00:00:00.01