如何使用 sql 或 codeigniter 3 querybuilder 连接 5 个表（4 个子表 + 1 个主表）？

Question

I have been building a hobby project where I want to write summaries about films, series, animes, tales etc. I want to join 4 tables to 1 main table, but I got a problem. I'm getting null values. At my database the data are exist. But I can not figure out what's the problem with my query.

The concept is as follows. There is an interface where I can record the data. After submitting the form, the controller decides what has been recorded. It saves in the main table what has been saved (anime, movie, etc.) and increases the autoincrement value. It then stores the identifier of the main table and other data (description, title, image, etc.) in one of the subtables. This works nicely. Then I wanted to list the contents of these 4 sub-tables, and here I ran into a problem.

Here is my query: function getContents() {

    $this->db->select('*');
    $this->db->from('contents'); 

    $this->db->join('films',  'films.content_id = contents.global_content_id', 'left');
    $this->db->join('series', 'series.content_id = contents.global_content_id', 'left');
    $this->db->join('tales',  'tales.content_id = contents.global_content_id', 'left');
    $this->db->join('animes', 'animes.content_id = contents.global_content_id', 'left');

    $query = $this->db->get();
    if ($query->num_rows()>0) return $query->result_array();
}

That's what I got back:

Array
(
    [0] => Array
        (
            [global_content_id] => 2
            [content_main_category] => film
            [id] => 
            [title] => 
            [description] => 
            [short_description] => 
            [category_id] => 
            [picture_path] => 
            [running_time] => 
            [director] => 
            [release_date] => 
            [language] => 
            [actors] => 
            [main_category_id] => 
            [uploader_id] => 
            [upload_time] => 
            [link] => 
            [status] => 
            [content_id] => 
            [subtitle_status] => 
            [subtitle_language] => 
        )

    [1] => Array
        (
            [global_content_id] => 3
            [content_main_category] => film
            [id] => 
            [title] => 
            [description] => 
            [short_description] => 
            [category_id] => 
            [picture_path] => 
            [running_time] => 
            [director] => 
            [release_date] => 
            [language] => 
            [actors] => 
            [main_category_id] => 
            [uploader_id] => 
            [upload_time] => 
            [link] => 
            [status] => 
            [content_id] => 
            [subtitle_status] => 
            [subtitle_language] => 
        )

Answer 1

Thank you guys for the help!

That's the solution what I wanted:

function getContents()
{
  $this->db->select('*');
  $this->db->from('films');
  $query1 = $this->db->get_compiled_select();
        
  $this->db->select('*');
  $this->db->from('animes');
  $query2 = $this->db->get_compiled_select();

  $this->db->select('*');
  $this->db->from('series');
  $query3 = $this->db->get_compiled_select();
        
  $this->db->select('*');
  $this->db->from('tales');
  $query4 = $this->db->get_compiled_select();
        
  $query = $this->db->query('SELECT * FROM ('.$query1 . ' UNION ALL ' . $query2 . ' UNION ALL ' . $query3 . ' UNION ALL ' . $query4.') AS bigtable LEFT JOIN ohf_contents ON ohf_contents.global_content_id = bigtable.content_id');

  if ($query->num_rows()>0) return $query->result_array();  


  /*       
    Pure SQL
        
    SELECT * FROM
    (SELECT * FROM ohf_films f UNION ALL
    SELECT * FROM ohf_animes a UNION ALL
    SELECT * FROM ohf_series s UNION ALL
    SELECT * FROM ohf_tales t) AS bigtable LEFT JOIN ohf_contents ON ohf_contents.global_content_id = bigtable.content_id;
  */
}