MySQL透视表与动态列的查询

Question

我正在使用下表存储产品数据：

mysql> SELECT * FROM product;
+---------------+---------------+--------+
| id | name     | description   | stock  |
+---------------+---------------+--------+
|  1 | product1 | first product |    5   | 
|  2 | product2 | second product|    5   | 
+---------------+---------------+--------+

mysql> SELECT * FROM product_additional;
+-----------------+------------+
| id | fieldname  | fieldvalue |
+-----------------+------------+
|  1 | size       | S          |
|  1 | height     | 103        |
|  2 | size       | L          |
|  2 | height     | 13         |
|  2 | color      | black      |
+-----------------+------------+

使用以下查询从两个表中选择记录

mysql> SELECT 
    p.id
    , p.name
    , p.description
    ,MAX(IF(pa.fieldname = 'size', pa.fieldvalue, NULL)) as `size`
    ,MAX(IF(pa.fieldname = 'height', pa.fieldvalue, NULL)) as `height`
    ,MAX(IF(pa.fieldname = 'color', pa.fieldvalue, NULL)) as `color`
FROM product p
LEFT JOIN product_additional AS pa ON p.id = pa.id
GROUP BY p.id
+---------------+---------------+--------+---------+--------+
| id | name     | description   | size   | height  | color  |
+---------------+---------------+--------+---------+--------+
|  1 | product1 | first product | S      | 103     | null   |
|  2 | product2 | second product| L      | 13      | black  |
+---------------+---------------+--------+---------+--------+

一切都正常工作:)

因为我动态地填充了“附加”表，所以如果查询也是动态的，那就很好了。这样，我不必每次输入新的字段名和字段值就更改查询。

Answer 1

MySQL中动态执行此操作的唯一方法是使用Prepared语句。这是一篇关于它们的好文章：

动态数据透视表（将行转换为列）

您的代码如下所示：

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'MAX(IF(pa.fieldname = ''',
      fieldname,
      ''', pa.fieldvalue, NULL)) AS ',
      fieldname
    )
  ) INTO @sql
FROM product_additional;

SET @sql = CONCAT('SELECT p.id
                    , p.name
                    , p.description, ', @sql, ' 
                   FROM product p
                   LEFT JOIN product_additional AS pa 
                    ON p.id = pa.id
                   GROUP BY p.id');

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

观看演示

注意：GROUP_CONCAT函数的限制为1024个字符。参见参数group_concat_max_len