Is the 'as' keyword required in Oracle to define an alias?

According to the select_list Oracle select documentation the AS is optional.

As a personal note I think it is easier to read with the AS

(Tested on Oracle 11g)

About AS:

• When used on result column, AS is optional.
• When used on table name, AS shouldn't be added, otherwise it's an error.

About double quote:

• It's optional & valid for both result column & table name.

e.g

-- 'AS' is optional for result column
select (1+1) as result from dual;
select (1+1) result from dual;


-- 'AS' shouldn't be used for table name
select 'hi' from dual d;


-- Adding double quotes for alias name is optional, but valid for both result column & table name,
select (1+1) as "result" from dual;
select (1+1) "result" from dual;

select 'hi' from dual "d";

AS without double quotations is good.

SELECT employee_id,department_id AS department
FROM employees
order by department

--ok--

SELECT employee_id,department_id AS "department"
FROM employees
order by department

--error on oracle--

so better to use AS without double quotation if you use ORDER BY clause