在Oracle中，

• 表别名不支持as关键字
• 列别名可以，但不是必需的

具有as的表别名：

SQL> select * from dept as newdept;
select * from dept as newdept
                   *
ERROR at line 1:
ORA-00933: SQL command not properly ended

不带as的表别名：

SQL> select * from dept newdept;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

组合：

SQL> select deptno as dno, dname department_name from dept newdept;
                   --            ----                      -------
    column's alias with AS       without AS             table alias without AS

       DNO DEPARTMENT_NAM
---------- --------------
        10 ACCOUNTING
        20 RESEARCH
        30 SALES
        40 OPERATIONS

SQL>