Codeigniter Group按最后记录

Question

假设我有3或4个表，其中一些表与父表连接。我想按最后记录显示小组。

Table: table1
--------------------------------------------------------------------
| table1Id(AI)(PK) |  date      | tagid | blah3 | blah3 | blah4 |
--------------------------------------------------------------------
1                  | 2016-05-01 | 101   |
2                  | 2016-05-04 | 102   |
3                  | 2016-05-10 | 101   |
4                  | 2016-05-15 | 101   |
5                  | 2016-05-04 | 103   |
6                  | 2016-05-20 | 101   |

但是当我按照tagid查询分组时，它将检索第一行

--------------------------------------------------------------------
| table1Id(AI)(PK) |  date      | tagid | blah3 | blah3 | blah4 |
--------------------------------------------------------------------
1                  | 2016-05-04 | 101   |
2                  | 2016-05-04 | 102   |
5                  | 2016-05-04 | 103   |

我希望看起来像这样

--------------------------------------------------------------------
| table1Id(AI)(PK) |  date      | tagid | blah3 | blah3 | blah4 |
--------------------------------------------------------------------
6                  | 2016-05-20 | 101   |
2                  | 2016-05-04 | 102   |
5                  | 2016-05-04 | 103   |

我的查询

$this->db->select('*');
$this->db->from('tablename1');
$this->db->join('tablename2', 'tablename2.tagid= tablename1.tagid', 'left');
$this->db->group_by('tablename1.tagId, tablename2.tagId');
$this->db->order_by('tablename1.tagId','asc'); 

Answer 1

使用子查询。见下面的代码

$this->db->select('*');
$this->db->from('tablename1');
$this->db->order_by('table1Id','DESC');
$subquery = $this->db->get_compiled_select();

$this->db->select('*');
$this->db->from('('.$subquery.') a');
$this->db->join('tablename2', 'tablename2.tagid= a.tagid', 'left');
$this->db->group_by('a.tagId, tablename2.tagId');
$this->db->order_by('a.tagId','asc');