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在查询时，可以为表和字段取一个别名。这个别名可以代替其指定的表和字段
为表取别名
mysql> select * from department d
    -> where d.d_id=1001;
+------+-----------+--------------+-------------+
| d_id | d_name    | function     | address     |
+------+-----------+--------------+-------------+
| 1001 | 科技部    | 研发产品       | 3号楼5层     |
+------+-----------+--------------+-------------+
 row in set (0.00 sec)
这里为department表取了一个别名为d
为字段取别名
mysql> select d_id as department_id, d_name as department_name
    -> from department;
+---------------+-----------------+
| department_id | department_name |
+---------------+-----------------+
|          1001 | 科技部          |
|          1002 | 生产部          |
|          1003 | 销售部          |
+---------------+-----------------+
 rows in set (0.00 sec)
此处语法与上面类似，只不过加了as
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