原文：http://blog.csdn.net/lwei_998/article/details/6093807

The UNION operator returns only distinct rows that appear in either result,

while the UNION ALL operator returns all rows.
The UNION ALL operator does not eliminate duplicate selected rows

union返回不重复行。 union all 返回所有行。

You have to use the Order By at the end of ALL the unions。
 the ORDER BY is considered to apply to the whole UNION result

(it's effectively got lower binding priority than the UNION). 
The ORDER BY clause just needs to be the last statement, after you've done all your unioning.
You can union several sets together, then put an ORDER BY clause after the last set.

ORDER BY  是针对整个union的结果集order by，只能用在union的最后一个子查询中。

SQL> SELECT employee_id, department_id
  2    FROM employees
  3   WHERE department_id = 50
  4   ORDER BY department_id
  5  UNION
  6  SELECT employee_id, department_id
  7    FROM employees
  8   WHERE department_id = 90
  9  ;

SELECT employee_id, department_id
  FROM employees
 WHERE department_id = 50
 ORDER BY department_id
UNION
SELECT employee_id, department_id
  FROM employees
 WHERE department_id = 90

ORA-00933: SQL 命令未正确结束

SQL>
SQL> SELECT employee_id, department_id
  2    FROM employees
  3   WHERE department_id = 50
  4  UNION
  5  SELECT employee_id, department_id
  6    FROM employees
  7   WHERE department_id = 90
  8    ORDER BY department_id;

EMPLOYEE_ID DEPARTMENT_ID
----------------   --------------------
        120                              50
        121                              90

也可以用在排序中使用列号，代替列名

SQL> SELECT employee_id, department_id
  2    FROM employees
  3   WHERE department_id = 50
  4  UNION
  5  SELECT employee_id, department_id
  6    FROM employees
  7   WHERE department_id = 90
  8    ORDER BY 2
  9  ;

EMPLOYEE_ID DEPARTMENT_ID
----------------   --------------------
        120                              50
        121                              90