MySQL Group By在多列上相等的值

Question

从示例开始：

对于这些数据：

NAME    Col1  Col2  Col3
row1    0     2     4
row2    1     5     0
row3    1     1     0

所需的查询输出是：

ColValue CountInCol1 CountInCol2 CountInCol3
0        1           0           2
1        2           1           0
2        0           1           0
4        0           0           1
5        0           1           0

问题解释如下：

如果列共享公共值，那么查询中返回的行如何在多列上压缩？在此示例中，如何将Col1，Col2和Col3分组到同一个ColValue结果中？

Answer 1

我会通过取消数据然后重新编译来完成此操作：

select colvalue,
       sum(which = 'col1') as CountInCol1,
       sum(which = 'col2') as CountInCol2,
       sum(which = 'col3') as CountInCol3
from (select 'col1' as which, col1 as colvalue from data union all
      select 'col2', col2 from data union all
      select 'col3', col3 from data
     ) d
group by colvalue;