MySQL选择返回虚拟列？

Question

有没有办法我可以做一个SELECT并告诉它返回3列和一个虚拟空列？我需要4列,第3列必须是'',直到表在数据库中的某个位置.我不允许向任何表添加任何列.

Answer 1

Select column1, column2, 'waiting for table' as dummy_column, column4
from your_table

Answer 2

mysql> SELECT '' AS empty_col;
+-----------+
| empty_col |
+-----------+
|           |
+-----------+
1 row in set (0.00 sec)

Answer 3

用于具有空字符串的列

Select column1, column2, '' as dummy_col, column 4 
  from your_table 

或者具有Null值的列

Select column1, column2, Null as dummy_col, column 4 
  from your_table