具有动态数据透视的Mysql查询

我有一个表名为grades，如下所示：

  +--------+--------+-----------+
  | userid | itemid | finalgrade|
  +--------+--------+-----------+
  |  1     |   7    |    100    | 
  |  1     |   8    |    89     |    
  |  2     |   7    |    87     |     
  |  2     |   8    |    17     |    
  |  3     |   7    |    87     |   
  |  3     |   8    |    38     |   
  +--------+--------+-----------+

  +--------+---+---+
  | userid | 7 | 8 |
  +--------+---+---+
  |  1     |100| 89| 
  |  2     |87 | 17|        
  |  3     |87 | 38|    
  +--------+---+---+

SELECT userid,
  MAX(CASE WHEN itemid=7 THEN finalgrade END) as c7,
  MAX(CASE WHEN itemid=8 THEN finalgrade END) as c8,
FROM grades
GROUP BY userid

SELECT    
  CONCAT(    
    'SELECT userid,',    
    GROUP_CONCAT(CONCAT('MAX(CASE WHEN itemid=', itemid, ' THEN finalgrade END) AS c', itemid)),    
    ' FROM tablename GROUP BY userid')    
FROM (SELECT DISTINCT itemid FROM tablename) s INTO @sql;

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

我想创建一个pivot查询，将表更改为：

  +--------+--------+-----------+
  | userid | itemid | finalgrade|
  +--------+--------+-----------+
  |  1     |   7    |    100    | 
  |  1     |   8    |    89     |    
  |  2     |   7    |    87     |     
  |  2     |   8    |    17     |    
  |  3     |   7    |    87     |   
  |  3     |   8    |    38     |   
  +--------+--------+-----------+

  +--------+---+---+
  | userid | 7 | 8 |
  +--------+---+---+
  |  1     |100| 89| 
  |  2     |87 | 17|        
  |  3     |87 | 38|    
  +--------+---+---+

SELECT userid,
  MAX(CASE WHEN itemid=7 THEN finalgrade END) as c7,
  MAX(CASE WHEN itemid=8 THEN finalgrade END) as c8,
FROM grades
GROUP BY userid

SELECT    
  CONCAT(    
    'SELECT userid,',    
    GROUP_CONCAT(CONCAT('MAX(CASE WHEN itemid=', itemid, ' THEN finalgrade END) AS c', itemid)),    
    ' FROM tablename GROUP BY userid')    
FROM (SELECT DISTINCT itemid FROM tablename) s INTO @sql;

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

我有一个解决方案，使用如下定义的itemid：

select *,
truncate (sum(finalgrade*(1-abs(sign(itemid-7)))),0) as 7,
truncate (sum(finalgrade*(1-abs(sign(itemid-8)))),0) as 8,
FROM grades
group by userid"

---

问题是，我需要相同的查询，但没有定义itemid，动态创建列。

我想您需要这样的查询：

  +--------+--------+-----------+
  | userid | itemid | finalgrade|
  +--------+--------+-----------+
  |  1     |   7    |    100    | 
  |  1     |   8    |    89     |    
  |  2     |   7    |    87     |     
  |  2     |   8    |    17     |    
  |  3     |   7    |    87     |   
  |  3     |   8    |    38     |   
  +--------+--------+-----------+

  +--------+---+---+
  | userid | 7 | 8 |
  +--------+---+---+
  |  1     |100| 89| 
  |  2     |87 | 17|        
  |  3     |87 | 38|    
  +--------+---+---+

SELECT userid,
  MAX(CASE WHEN itemid=7 THEN finalgrade END) as c7,
  MAX(CASE WHEN itemid=8 THEN finalgrade END) as c8,
FROM grades
GROUP BY userid

SELECT    
  CONCAT(    
    'SELECT userid,',    
    GROUP_CONCAT(CONCAT('MAX(CASE WHEN itemid=', itemid, ' THEN finalgrade END) AS c', itemid)),    
    ' FROM tablename GROUP BY userid')    
FROM (SELECT DISTINCT itemid FROM tablename) s INTO @sql;

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

但您希望它具有动态值，然后可以使用这样一个准备好的语句：

  +--------+--------+-----------+
  | userid | itemid | finalgrade|
  +--------+--------+-----------+
  |  1     |   7    |    100    | 
  |  1     |   8    |    89     |    
  |  2     |   7    |    87     |     
  |  2     |   8    |    17     |    
  |  3     |   7    |    87     |   
  |  3     |   8    |    38     |   
  +--------+--------+-----------+

  +--------+---+---+
  | userid | 7 | 8 |
  +--------+---+---+
  |  1     |100| 89| 
  |  2     |87 | 17|        
  |  3     |87 | 38|    
  +--------+---+---+

SELECT userid,
  MAX(CASE WHEN itemid=7 THEN finalgrade END) as c7,
  MAX(CASE WHEN itemid=8 THEN finalgrade END) as c8,
FROM grades
GROUP BY userid

SELECT    
  CONCAT(    
    'SELECT userid,',    
    GROUP_CONCAT(CONCAT('MAX(CASE WHEN itemid=', itemid, ' THEN finalgrade END) AS c', itemid)),    
    ' FROM tablename GROUP BY userid')    
FROM (SELECT DISTINCT itemid FROM tablename) s INTO @sql;

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

---

您可以使用SUM而不是MAX，这取决于数据的结构。

谢谢您的解决方案，但查询有语法错误：

您的SQL语法有错误；查看与您的MySQL服务器版本对应的手册，了解使用near“PREPARE stmt FROM@sql；执行stmt；在第8行取消分配PREPARE stmt。

@EnriqueBerrueta对我来说是正确的：itemid是INT列吗？是的，itemid是INT。我使用的是SqlFIDLE，这让我很惊讶，因为它是正确的，问题是当我在phpmyadmin或php中使用它时。@EnriqueBerrueta您有很多不同的itemid吗？group_concat有一个长度限制，我怀疑查询可能已被剪切。。。但是可以使用

set group_concat_max_len=”修改此限制默认值为1024，您可以尝试使用10000…是的，这对我有一点帮助，但现在查询给了我另一个错误：
Unknown prepared statement handler（stmt）given to EXECUTE