Table Alias for a single table query

Question

Hi,
What is the difference between using table alias and not using for a query involving single table.
Say
select empno, ename from emp where empno = 1000;
select e.empno,e.ename from emp where e.empno = 1000;
select 1 from emp e where e.empno = 1000;
how above three queries shall be parsed/executed. Can anyone please explain the process ......

Answer 1

Since the 3rd query doesn't access any column values the execution differs from the others:
SQL>  explain plan for select empno, ename from emp where empno = 1000
Explain complete.
SQL>  select * from table(dbms_xplan.display)
PLAN_TABLE_OUTPUT                                                                                  
Plan hash value: 2949544139                                                                        
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |             
|   0 | SELECT STATEMENT            |        |     1 |    10 |     1   (0)| 00:00:01 |             
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     1 |    10 |     1   (0)| 00:00:01 |             
|*  2 |   INDEX UNIQUE SCAN         | PK_EMP |     1 |       |     0   (0)| 00:00:01 |             
Predicate Information (identified by operation id):                                                
   2 - access("EMPNO"=1000)                                                                        
14 rows selected.
SQL>  truncate table plan_table
Table truncated.
SQL>  explain plan for select 1 from emp e where e.empno = 1000
Explain complete.
SQL>  select * from table(dbms_xplan.display)
PLAN_TABLE_OUTPUT                                                                                  
Plan hash value: 56244932                                                                          
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |                       
|   0 | SELECT STATEMENT  |        |     1 |     4 |     0   (0)| 00:00:01 |                       
|*  1 |  INDEX UNIQUE SCAN| PK_EMP |     1 |     4 |     0   (0)| 00:00:01 |                       
Predicate Information (identified by operation id):                                                
   1 - access("E"."EMPNO"=1000)                                                                    
13 rows selected.