Mysql 根据存储在另一个表中的列名选择列

我试图从另一个表（UserTable）中获取单元格值，原始表（DataTable）中为该表指定了单元格的列。单元格值将用于计算一个分数，该分数将决定查询结果的顺序

UserTable:
+--------+------+------+------+
| userid | c1   | c2   | c3   |
+--------+------+------+------+
|    id1 |    0 |    1 |    1 |
|    id2 |    0 |    0 |    1 |
|    id3 |    1 |    0 |    0 |
|    id4 |    1 |    1 |    0 |
+--------+------+------+------+

DataTable:
+--------+------+------+------+
| id     | ColA | ColB | ColC |
+--------+------+------+------+
|      1 | A    | B    | c1   |
|      2 | D    | E    | c3   |
|      3 | G    | H    | c2   |
|      4 | J    | K    | c3   |
+--------+------+------+------+

我当前未工作的查询是：

SELECT *, 
       (SELECT DataTable.ColC 
        FROM UserTable 
        WHERE UserTable.userid = 'some_user_id') AS score 
FROM DataTable 
ORDER BY score DESC

但这不会在UserTable中选择适当的列，而是返回ColC中包含的字符串值

预期产出： 例如，如果'some_user_id'设置为'id2'。预期产出将是：

+--------+------+------+------+------+
| id     | ColA | ColB | ColC | score|
+--------+------+------+------+------+
|      2 | D    | E    | c3   |  1   |
|      4 | J    | K    | c3   |  1   |
|      1 | A    | B    | c1   |  0   |
|      3 | G    | H    | c2   |  0   |
+--------+------+------+------+------+

其中score列只是从UserTable中查找值

---

有可能解决此问题吗？

您不能动态地执行此操作。但您可以构建将列名映射到列的CASE语句：

SELECT DataTable.*, (
    SELECT CASE DataTable.ColC
        WHEN 'c1' THEN UserTable.c1
        WHEN 'c2' THEN UserTable.c3
        WHEN 'c3' THEN UserTable.c3
    END
    FROM UserTable
    WHERE UserTable.userid = 'id2'
) AS score
FROM DataTable
ORDER BY score DESC

---

我想通过取消激活用户表来表达这一点：

select dt.*, uts.score
from datatable dt left join
     ((select ut.userid, 'c1' as which, c1 as score
       from usertable ut
      ) union all
      (select ut.userid, 'c2' as which, c2 as score
       from usertable ut
      ) union all
      (select ut.userid, 'c3' as which, c3 as score
       from usertable ut
      ) 
     ) uts
     on dt.colc = uts.which 
where uts.userid = ?;

---

我之所以喜欢这种方法，是因为子查询建议了您真正需要的表。您不应该在列中存储不同的分数值。您应该有一个

userScores

表，每个用户和每个分数有一行。

我现在刚刚更新了它。非常感谢您的解决方案，我非常感谢！虽然这有点烦人，但由于我的专栏随着时间的推移而发生变化，所以无法动态完成。谢谢您的回复。您是否测试过此代码/您使用的mysql版本是什么？在5.6上，语法不签出。例如，前两个Declare语句是否应该改为SET语句？您提出了一个很好的观点。我想我将为每个userid创建一个表，每个列（c1、c2、c3）都有一行以及相应的分数。对于我的应用程序，有数千个名为c1..cN的列

select dt.*, uts.score
from datatable dt left join
     ((select ut.userid, 'c1' as which, c1 as score
       from usertable ut
      ) union all
      (select ut.userid, 'c2' as which, c2 as score
       from usertable ut
      ) union all
      (select ut.userid, 'c3' as which, c3 as score
       from usertable ut
      ) 
     ) uts
     on dt.colc = uts.which 
where uts.userid = ?;