MySQL Pivot Table Query: Using Dynamic Columns for Data Analysis

Question

I am using the following table to store product data:

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 |
 ------------------ ------------ 

Use the following query to select records from both tables:

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 |
 --------------- --------------- -------- ---------- --- ----- 

Everything works fine :)

Since I am populating the "additional" table dynamically, it would be nice if the query was also dynamic. This way I don't have to change the query every time I enter a new field name and field value.

Answer 1

I have a slightly different way of accomplishing this than the accepted answer. This approach avoids using GROUP_CONCAT which has a 1024 character limit by default and won't work if you have a lot of fields unless you change the limit.

SET @sql = '';
SELECT
    @sql := CONCAT(@sql,if(@sql='','',', '),temp.output)
FROM
(
    SELECT
      DISTINCT
        CONCAT(
         'MAX(IF(pa.fieldname = ''',
          fieldname,
          ''', pa.fieldvalue, NULL)) AS ',
          fieldname
        ) as output
    FROM
        product_additional
) as temp;

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, p.name, p.description');

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

Click here to view the demo

Answer 2

In MySQL, the only way to do this dynamically is to use prepared statements. Here is a good article about them:

Dynamic pivot table (convert rows to columns)

Your code will look like this:

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, p.name, p.description');

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

ViewDemo

Note: The character limit of the GROUP_CONCAT function is 1024 characters. See parameter group_concat_max_len