Codeigniter将2个表数据与id值的新列连接起来

Question

I'm new to Codeigniter and PHP, just learning. I've searched for this but couldn't find the right answer. I have a table that contain some data, lets call it posts and in this table, each post have a category_id. I have another table called categories and in this table I have 2 columns: id and name.

I want to join the two tables that the result I'll get is the same as posts with another column called category_name that will be taken from categories.name. Just can't figure out how to do so.

This is what I was up to so far:

function getPostsWithByCategoryID($numberOfRows, $start, $categoryId)
{
    $this->db->select('*');
    $this->db->from('posts')->order_by('id','desc')->limit($numberOfRows, $start);
    $this->db->where('category_id', $categoryID);
    $this->db->join('categories', 'posts.category_id == categories.id');
    $query = $this->db->get();
    return $query->result_array();
}

Thanks in advance.

========================================================================

EDIT:

After trying Adrian Forsius answer:

function getPostsWithByCategoryID($numberOfRows, $start, $categoryId)
{
    $this->db->select('*, category.name AS category_name');
    $this->db->from('posts')->order_by('id','desc')->limit($numberOfRows, $start);
    $this->db->where('category_id', $categoryID);
    $this->db->join('categories', 'posts.category_id = categories.id');
    $query = $this->db->get();
    return $query->result_array();
}

I'm getting this error:

Error Number: 1054: Unknown column 'category.name' in 'field list'
SELECT *, `category`.`name` AS category_name
FROM (`posts`)
JOIN `categories` ON `posts`.`category_id` = `categories`.`id`
WHERE `category_id` IS NULL
ORDER BY `id` desc
LIMIT 20

====

EDIT 2:

So I found some mistakes in the code and fixed them, tried again, but still I get an error: (The mistakes were using $categoryID instead of $categoryId and select category.name instead of the true name of the table categories which is categories.name

Error Number: 1052: Column 'id' in order clause is ambiguous
SELECT *, `categories`.`name` AS category_name
FROM (`posts`)
JOIN `categories` ON `posts`.`category_id` = `categories`.`id`
WHERE `category_id` =  '3'
ORDER BY `id` desc
LIMIT 20

Answer 1

This should do what you are looking for:

function getPostsWithByCategoryID($numberOfRows, $start, $categoryId)
{
    $this->db->select('*, categories.name AS category_name');
    $this->db->from('posts')->order_by('posts.id','desc')->limit($numberOfRows, $start);
    $this->db->where('category_id', $categoryId);
    $this->db->join('categories', 'posts.category_id = categories.id');
    $query = $this->db->get();
    return $query->result_array();
}