例如:现在有department表，如下：

mysql> select * from department;
+------+-----------+--------------+-------------+
| d_id | d_name    | function_    | address     |
+------+-----------+--------------+-------------+
| 1001 | 科研部    | 研发产品     | 3号楼5层    |
| 1002 | 生产部    | 生产产品     | 5号楼1层    |
| 1003 | 销售部    | 策划销售     | 5号楼1层    |
+------+-----------+--------------+-------------+
3 rows in set (0.00 sec)

mysql> select d_name as department_name from department where
    -> department_name = 销售部;
ERROR 1054 (42S22): Unknown column 'department_name' in 'where clause'

对比地，表别名却可以在查询说中使用，例如：

mysql> select * from department d where d.d_id = 1003;
+------+-----------+--------------+-------------+
| d_id | d_name    | function_    | address     |
+------+-----------+--------------+-------------+
| 1003 | 销售部    | 策划销售     | 5号楼1层    |
+------+-----------+--------------+-------------+
1 row in set (0.00 sec)

总结,mysql字段别名在显示的时候用，查询时候不能用，表别名在查询时候用，在显示的时候不用。