使用codeigniter按每个类别的降序获取三条记录

Question

我有两个类别，我想获取每个类别的三个记录，稍后我找到了这个链接UNION query with codeigniter's active record pattern，之后我更改了我的DB_Active_rec文件，并添加了以下代码

var $unions = array();

public function union_push($table = '') {
    if ($table != '') {
        $this->_track_aliases($table);
        $this->from($table);
    }

    $sql = $this->_compile_select();

    array_push($this->unions, $sql);
    $this->_reset_select();
}

public function union_flush() {
    $this->unions = array();
}

public function union() {
    $sql = '(' . implode(') union (', $this->unions) . ')';
    $result = $this->query($sql);
    $this->union_flush();
    return $result;
}

public function union_all() {
    $sql = '(' . implode(') union all (', $this->unions) . ')';
    $result = $this->query($sql);
    $this->union_flush();
    return $result;
}

然后创建基于codeigniter函数的查询，如下所示

$this->db->select("*");
$this->db->from("media m");
$this->db->join("category c", "m.category_id=c.id", "INNER");
$this->db->order_by("m.media_files", "DESC");
$this->db->limit(3);
$this->db->union_push();
$this->db->select("*");
$this->db->from("media m");
$this->db->join("category c", "m.category_id=c.id", "INNER");
$this->db->order_by("m.media_files", "DESC");
$this->db->limit(3);
$this->db->union_push();
$getMedia = $this->db->union_all();

创建此

(SELECT * FROM media m INNER JOIN category c ON 
m.category_id = c.id ORDER BY m.media_files DESC LIMIT 3)
UNION ALL
(SELECT * FROM media m INNER JOIN category c ON 
m.category_id = c.id ORDER BY m.media_files DESC LIMIT 3)

现在它正在获取记录，但不正确我只想使用查询，它显示六个记录第一个查询获取3个记录，第二个查询获取三个记录现在记录是重复的我检查记录的id是6,5,4和6,5,4。这也可以通过PHP完成，但我想使用查询。提前感谢

Answer 1

我不知道代码触发器，但基本上你想让它先做并集，然后在整个集合上应用order by。这将需要一个子查询。它应该产生以下SQL查询：

select * from
    ((SELECT * FROM media m INNER JOIN category c ON m.category_id = c.id )
    UNION ALL
    (SELECT * FROM media m INNER JOIN category c ON m.category_id = c.id)) T
ORDER BY m.media_files DESC LIMIT 3

希望能对你有所帮助。