mysql: select rows from another table as columns

Question

is it somehow possible in mysql to get rows from a second table (content) to be displayed as columns in the first one (inventory)?

As the ID in the second table is not unique a JOIN produces duplicates, but i need all elements of the first table as a single row.

these are the tables: http://sqlfiddle.com/#!9/6da06/1

a result like this is needed:

id name      location   desc1  level1  desc2  level2  desc3  level3
1  test      somewhere  abc    20      def    50      ghi    30
2  anything  something  rfg    20      lzb    80      null   null
3  xxyzyzy   dffsdfd    atc    20      null   null    null   null

changes to the table structures are not possible, so is there any way how mysql can do that. there is no limit on how much duplicates of id can occur in table content... every row of inventory is needed, no duplicates. would be very cool, if sql can do something, or merging these arrays in PHP ?

Answer 1

If there is a limited number of content rows associated with each inventory row, then you can use the following query:

SELECT i.`id`, i.`name`, i.`location`,
       MAX(CASE WHEN c.rn = 1 THEN c.`desc` END) AS desc1,
       MAX(CASE WHEN c.rn = 1 THEN c.`level` END) AS level1,
       MAX(CASE WHEN c.rn = 2 THEN c.`desc` END) AS desc2,
       MAX(CASE WHEN c.rn = 2 THEN c.`level` END) AS level2,
       MAX(CASE WHEN c.rn = 3 THEN c.`desc` END) AS desc3,
       MAX(CASE WHEN c.rn = 3 THEN c.`level` END) AS level3
FROM inventory AS i
LEFT JOIN ( 
  SELECT `id`, `desc`, `level`,
         @row_number := IF (@id <> id, 
                           IF (@id := id, 1, 1),
                           IF (@id := id, @row_number+1, @row_number+1)) AS rn
  FROM content 
  CROSS JOIN (SELECT @row_number := 0, @id := 0) AS vars
  ORDER BY `id`, `desc` 
) AS c ON i.`id` = c.`id`
GROUP BY i.id, i.name, i.location  

The above uses variables to assign incremental numbers to rows of content that belong to the same id slice. Conditional aggregation is then used to pivot correlated content rows.

Demo here

Answer 2

Hey try this for pivot of table it will generate your result with column name as value of your level concat with 'description' and 'level' here you have no needs to detect length of level ie level1,level2. this might helps you SQLFIDDLE

set @sql = null;
set @sql1 = null;

select
  group_concat(distinct
    concat(
      'max(case when c.level = ''',
      c.level,
      ''' then c.level end) AS ',
      concat('level','_',level)
    )
  ) into @sql
  from inventory i left join content c on i.id = c.id;

  select
  group_concat(distinct
    concat(
      'max(case when c.level = ''',
      c.level,
      ''' then c.desc end) AS ',
      concat('desc','_',level)
    )
  ) into @sql1
  from inventory i left join content c on i.id = c.id;

set @sql = concat('select i.Name,', @sql, ',',@sql1,' from inventory i left join content c on i.id = c.id group by i.id
');

prepare stmt from @sql;
execute stmt;
deallocate prepare stmt;