Question

I have a table:

CREATE TABLE `Issues` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `title` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

I have another table:

CREATE TABLE `Attachments` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `issue_id` int(11) DEFAULT NULL,
  `attachment` text,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

How can I get the data to look like this:

issue_id    title    attachment1    attachment2    attachment3
--------------------------------------------------------------
1           T1       a1.png         a2.png
2           T2
3           T3       b4.gif         xyz.doc        ttt.file

The problem I can’t figure out is how to get the dynamic set of attachments into a dynamic column grouped by issue. I have determined that the maximum amount of attachments for one issue is 12, but the total per ticket can be anywhere from 0-12. I’m stumped…

I’ve tried this
MySQL pivot row into dynamic number of columns, but can’t make sense of it in my case because I’m building dynamic columns based on total matches per record…

Any help would be greatly appreciate. Please let me know if this doesn’t make sense.

Nino

Answer 1

Here is one way to accomplish this if you know the max number is 12. It uses MAX and CASE, getting the Row Number for each attachment.

SELECT 
    I.Id issue_id, 
    I.title, 
    MAX(CASE WHEN d.row_number = 1 THEN D.attachment END) attachment1,
    MAX(CASE WHEN d.row_number = 2 THEN D.attachment END) attachment2,
    MAX(CASE WHEN d.row_number = 3 THEN D.attachment END) attachment3,
    MAX(CASE WHEN d.row_number = 4 THEN D.attachment END) attachment4,
    MAX(CASE WHEN d.row_number = 5 THEN D.attachment END) attachment5,
    MAX(CASE WHEN d.row_number = 6 THEN D.attachment END) attachment6,
    MAX(CASE WHEN d.row_number = 7 THEN D.attachment END) attachment7,
    MAX(CASE WHEN d.row_number = 8 THEN D.attachment END) attachment8,
    MAX(CASE WHEN d.row_number = 9 THEN D.attachment END) attachment9,
    MAX(CASE WHEN d.row_number = 10 THEN D.attachment END) attachment10,
    MAX(CASE WHEN d.row_number = 11 THEN D.attachment END) attachment11,
    MAX(CASE WHEN d.row_number = 12 THEN D.attachment END) attachment12
FROM Issues I
  LEFT JOIN (
  SELECT 
      a.issue_id, 
      @running:=if(@previous=a.issue_id,@running,0) + 1 as row_number,
      @previous:=a.issue_id,
      a.attachment
  FROM Attachments a
      JOIN    (SELECT @previous := 0) r
    ORDER BY a.issue_id, a.attachment
  ) D ON I.ID = D.issue_id
GROUP BY I.Id, I.Title

And here is the SQL Fiddle.

I had to make an edit to make the rownumber reset per group. Should be working now. Also, per @spencer7593’s great comment, I’ve updated the query slightly.

–EDIT–

In response to OP’s comment about needing dynamic results, this should work:

SET @sql = NULL;

SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'MAX(IF(d.row_number = ', d.row_number, ',D.attachment,NULL)) AS attachment', d.row_number)
  ) INTO @sql
FROM Issues I
  LEFT JOIN (
  SELECT 
      a.issue_id, 
      @running:=if(@previous=a.issue_id,@running,0) + 1 as row_number,
      @previous:=a.issue_id,
      a.attachment
  FROM Attachments a
      JOIN    (SELECT @previous := 0) r
    ORDER BY a.issue_id, a.attachment
  ) D ON I.ID = D.issue_id
;

SET @sql = CONCAT('SELECT I.Id issue_id, 
                          I.title, ', @sql, ' 
                  FROM Issues I
                  LEFT JOIN (
                  SELECT 
                      a.issue_id, 
                      @running:=if(@previous=a.issue_id,@running,0) + 1 as row_number,
                      @previous:=a.issue_id,
                      a.attachment
                  FROM Attachments a
                      JOIN    (SELECT @previous := 0) r
                    ORDER BY a.issue_id, a.attachment
                  ) D ON I.ID = D.issue_id
                GROUP BY I.Id, I.Title');

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

And here is the SQL Fiddle.