How to fetch the records which is belongs to only one group in postgresql I want to fetch the student_id which is only belongs to one collage 
expected op 
6 and 7 stud_id's so how can i do this?

 _id | stud_id | collage_id | university_id  | active 
-----+---------+------------+----------------+--------
   1 |       2 |          2 |              2 | t
   6 |       2 |          3 |              3 | t
   7 |       6 |          2 |              2 | t
   8 |       7 |          2 |              2 | t
   9 |       8 |          2 |              2 | t
  10 |       8 |          3 |              2 | t


and also if i need fetch the data which is belongs to perticular collage , i mean to use where clause ? thank you
You could use count(distinct collage_id) grouping by stud_id in this way:

select   stud_id
from     tbl
group by stud_id
having   count(distinct collage_id) = 1;






| stud_id |
| ------: |
|       6 |
|       7 |



db&lt;>fiddle here

If you want to fetch rows filtering by collage_id:

select *
from   tbl
where  collage_id = 3
and    stud_id in (select   stud_id
                   from     tbl
                   group by stud_id
                   having   count(distinct collage_id) = 1);


Note I've modified a bit your sample data.





_id | stud_id | collage_id | university_id | active
--: | ------: | ---------: | ------------: | :-----
  8 |       7 |          3 |             2 | t     



db&lt;>fiddle here