Question

Ok, I’m still a beginner in sql and can not figure this one out yet.
I have four tables: companies, persons, details, person_details.

companies: 
id, compname
(1, ACME), 
(2, ACME Group), ...

persons: 
id, name, lastname, company id
(1, donald, duck, 1), 
(2, lucky, luke, 1), 
(3, mickey, mouse, 2)

details: 
id, description
(1, 'weight'), 
(2, 'height'), 
(3, 'haircolor'), ...

person_details: 
id, persons id, details id, value
(1, 1, 1, 70), 
(2, 1, 3, 'red'), 
(3, 2, 1, 90), 
(4, 3, 2, 180)

As you can see, not all persons have all the details and the list of available details is variable.

Now, for a given arary of person ids and detail ids, I would like to get rows containing: company name and id, person name and last name, detail name and value for each of the details in the supplied array.
Let’s say persons(1,2), details(1,3) should result in:

companies.id, companies.name, name, lastname, details.description, person_details.value,...    
    1, ACME, donald, duck, 'weight', 70, 'haircolor', 'red'
    2, ACEM, lucky, luke, 'weight', 90, 'haircolor', null

Help, please…

Answer 1

Based on your description it seems like you want to pivot the data but unfortunately MySQL does not have a pivot function so you will need to replicate it using an aggregate function with a CASE statement.

If you know the description values ahead of time you can hard-code your query to the following:

select c.id,
  c.compname,
  p.name,
  p.lastname,
  max(case when d.description = 'weight' then pd.value end) weight,
  max(case when d.description = 'haircolor' then pd.value end) haircolor,
  max(case when d.description = 'height' then pd.value end) height
from companies c
left join persons p
  on c.id = p.`company id`
left join person_details pd
  on p.id = pd.`persons id`
left join details d
  on pd.`details id` = d.id
-- where p.id in (1, 2)
group by c.id, c.compname, p.name, p.lastname

See SQL Fiddle with Demo

If you have an unknown number of values, then you can use a prepared statement to generate this dynamically similar to this:

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'MAX(CASE WHEN d.description = ''',
      description,
      ''' then pd.value end) AS ',
      description
    )
  ) INTO @sql
FROM details;

SET @sql = CONCAT('SELECT c.id,
                      c.compname,
                      p.name,
                      p.lastname, ', @sql, ' 
                  from companies c
                  left join persons p
                    on c.id = p.`company id`
                  left join person_details pd
                    on p.id = pd.`persons id`
                  left join details d
                    on pd.`details id` = d.id
                  -- where p.id in (1, 2)
                  group by c.id, c.compname, p.name, p.lastname');

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

See SQL Fiddle with Demo

Both versions generate the result:

| ID |   COMPNAME |   NAME | LASTNAME | WEIGHT | HEIGHT | HAIRCOLOR |
---------------------------------------------------------------------
|  1 |       ACME | donald |     duck |     70 | (null) |       red |
|  1 |       ACME |  lucky |     luke |     90 | (null) |    (null) |
|  2 | ACME Group | mickey |    mouse | (null) |    180 |    (null) |