mysql - 如何将所有列合并为一列

Question

我有一个看起来像这样的表:

+------+------+------+
| Col1 | Col2 | Col3 |
+------+------+------+
|    1 |    2 |    3 |
|    2 |    3 |    4 |
|    3 |    4 |    5 |
+------+------+------+

我想写一个查询来得到这个:

+------+
| Col1 |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|    5 |
+------+

我的问题是:是否可以在不使用 union 的情况下将所有列合并为一个列？

Answer 1

嗯，是的。但它并不优雅:

select distinct (case n when 1 then col1 when 2 then col2 when 3 then col3) as col
from t cross join
     (select (@rn := @rn + 1) as n
      from t cross join
           (select @rn := 0) params
      limit 3
     ) n;