Join tables with comma values

Question

I have a hard nut to crack with joing 3 tables. I have a newsletter_items, newsletter_fields and newsletter_mailgroups which I want to be joined to get a list of newsletters

Answer 1

I recommend that you make your joins explicit.
It makes it easier to debug your query and to change inner with left joins.
There is absolutely never a good reason to use SQL '89 implicit join syntax.

SELECT ni.*
       , nf.*
       , group_concat(nm.mailgroup_name) as mailgroups
FROM newsletter_items ni
INNER JOIN newsletter_fields nf 
  ON (nf.field_letter_uid = ni.letter_id)
INNER JOIN newsletter_mailgroups nm  
  ON (find_in_set(nm.mailgroup_id, ni.receivers))
WHERE  
  nf.field_name = 'letter_headline' 
  ni.template = '". $template ."' 
GROUP BY ni.letter_id;

Regarding your database design.
I recommend you normalize your database, that means that you move the comma separated fields into a different table.

So you make a table receivers

Receivers
----------
id integer auto_increment primary key
letter_id integer not null foreign key references newsletter_items(letter_id)
value integer not null

You then remove the field receiver from the table newsletter_items

Your query then changes into:

SELECT ni.*
       , group_concat(r.value) as receivers
       , nf.*
       , group_concat(nm.mailgroup_name) as mailgroups

FROM newsletter_items ni
INNER JOIN newsletter_fields nf 
  ON (nf.field_letter_uid = ni.letter_id)
INNER JOIN newsletter_mailgroups nm  
  ON (find_in_set(nm.mailgroup_id, ni.receivers))
LEFT JOIN receiver r ON (r.letter_id = ni.letter_id)
WHERE  
  nf.field_name = 'letter_headline' 
  ni.template = '". $template ."' 
GROUP BY ni.letter_id;

This change should also speed up your query significantly.