i have used the following query to select rows from tables.

Table 1:

id  description status add_date   topicid 
 1   xyz         0      22-3-13     5   
 2   pqr         0      21-3-13     5
 3   abc         0      20-3-13     5
 4   sdd         0      22-3-13     5


Table2:

id     otherid    
1       2  
2       3

This query gives me all the record from table1 but i want to select those record which is not in table2.
like table1 'id' not present in table2 'otherid'.
In my case want to select record from table1 for id 1 and 4.because which is not present in table2 as 'otherid'.

$topicid = 5;

$q =$this->db->select(array(
            't1.id as id',
            't1.description',
            't1.topicid',
            't1.add_date'))
            ->from('table1 AS t1')
            ->where('t1.topicid',$topicid)
            ->where('t1.status',0)
            ->order_by('t1.add_date DESC)->get();