MySQL select specific row values as column names

Question

Here is my basic participants table (displayed as associative array):

[id] => 1
[campaign_id] => 41
[firstname] => Jeff
[lastname] => Berube

On another table, named participants_custom, I can add multiple custom data, that belongs to a participants row. Like this:

[id] => 51
[participant_id] => 1
[name] => textfield_bh423vjhgv
[data] => qwerty1

[id] => 52
[participant_id] => 1
[name] => textfield_IDRr2kzjZR59Xjw
[data] => qwerty2

[id] => 53
[participant_id] => 1
[name] => textfield_6kj5bhjjg
[data] => qwerty3

I am currently making a join, but it adds the only name, participant_idand data to my row. What I want is my query to return something like this:

[id] => 1
[campaign_id] => 41
[firstname] => Jeff
[lastname] => Berube
[textfield_bh423vjhgv] => qwerty1
[textfield_IDRr2kzjZR59Xjw] => qwerty2
[textfield_6kj5bhjjg] => qwerty3

Where row value name becomes a column, and value, it's value. How can I make it?

I found this and this, which was unsuccessful. I'm finding a way that would work with my situation. Thanks for any help.

John Woo · Accepted Answer · 2013-04-12 17:44:33Z

2

SELECT  a.ID,
        a.Campaign_ID,
        a.FirstName,
        a.LastName,
        MAX(CASE WHEN b.data = 'qwerty1' THEN b.Name END) qwerty1,
        MAX(CASE WHEN b.data = 'qwerty2' THEN b.Name END) qwerty2,
        MAX(CASE WHEN b.data = 'qwerty3' THEN b.Name END) qwerty3
FROM    Participants a
        INNER JOIN Participants_Custom b
            ON a.ID = b.Participant_ID
GROUP   BY  a.ID,
            a.Campaign_ID,
            a.FirstName,
            a.LastName

SQLFiddle Demo

UPDATE 1

Since the values of data are unknown, a dynamic sql is much preferred.

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'MAX(CASE WHEN b.data = ''',
      data,
      ''' THEN b.Name ELSE NULL END) AS ',
      CONCAT('`',data, '`')
    )
  ) INTO @sql
FROM Participants_Custom;

SET @sql = CONCAT('SELECT  a.ID,
                           a.Campaign_ID,
                           a.FirstName,
                           a.LastName,', @sql, 
                  'FROM     Participants a
                            INNER JOIN Participants_Custom b
                                ON a.ID = b.Participant_ID
                    GROUP   BY  a.ID,
                                a.Campaign_ID,
                                a.FirstName,
                                a.LastName');

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

SQLFiddle Demo

edited Apr 12, 2013 at 17:44

answered Apr 12, 2013 at 17:34

John Woo

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3 Comments

Jeff B. Over a year ago

But b.data can be anything, not only qwerty1, querty2 or qwerty 3. How to if whatever the value in b.Name, is had to become a colum and have value as it's value?

Jeff B. Over a year ago

And number of rows in participants_custom is variable, between 0 and 6 most of time.

John Woo Over a year ago

then you need a dynamic sql, please wait :D

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