Just changed "a.ename, a.empno" into "a.*"

demo@ORA11G> create table emp as select * from scott.emp;

Table created.

demo@ORA11G> create table dept as select * from scott.dept;

Table created.

demo@ORA11G> exec dbms_stats.gather_table_stats(user,'dept');

PL/SQL procedure successfully completed.

demo@ORA11G> exec dbms_stats.gather_table_stats(user,'emp');

PL/SQL procedure successfully completed.

demo@ORA11G> set autotrace traceonly explain
demo@ORA11G> select a.*
  2   from dept a
  3   where exists(select null from emp b
  4   where a.deptno=b.deptno(+)) ;

Execution Plan
----------------------------------------------------------
Plan hash value: 1754319153

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     3 |    69 |     6   (0)| 00:00:01 |
|*  1 |  HASH JOIN SEMI    |      |     3 |    69 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    80 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP  |    14 |    42 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("A"."DEPTNO"="B"."DEPTNO")

With no relationship between "emp" and "dept" - in the above query the outer join is eliminated by Optimizer.

So the equivalent ANSI SQL would be this.

demo@ORA11G> select *
  2  from dept d
  3  where d.deptno in (select e.deptno from emp e ) ;

Execution Plan
----------------------------------------------------------
Plan hash value: 1754319153

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     3 |    69 |     6   (0)| 00:00:01 |
|*  1 |  HASH JOIN SEMI    |      |     3 |    69 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    80 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP  |    14 |    42 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("D"."DEPTNO"="E"."DEPTNO")