Couldn't get the exact set of data from multiple tables when i write a mysql query

I wrote a MySQL query to list some outlets which are in my tables and for some outlets, I have assigned some staffs and I need to list the outlets with its details and the assigned staff name and other details. I add my query below

SELECT a.outlet_id
     , a.outlet_name
     , a.owner_name
     , a.address
     , a.pin
     , a.outlet_place
     , a.contact_no
     , a.img
     , a.status
     , b.name
     , b.phone
     , b.id
     , (SELECT SUM(total_amount) FROM tbl_orders WHERE status = "delivered" AND outlet_id = a.outlet_id AND a.status = "active") totalamount 
  FROM tbl_outlet a
     , tbl_staff b
     , tbl_staff_to_outlet c 
 WHERE b.id = c.staff_id 
   AND a.outlet_id = c.outlet_id 
   AND b.status = "active" 
 GROUP 
    BY a.outlet_id

the problem is I got the list of outlet and details which are assigned to staff only. it didn't list out the outlets which are not assigned to any staff.

I have one table with outlet details named "tbl_outlet" and another table named "tbl_staff" which have staffs details and another table "tbl_staff_to_outlet" which have the outlet id and staff id to map which outlet is assigned to which staff.

I would suggest you to use ANSI JOIN syntax for writing JOINS. It provides more clarity.
You have to use OUTER JOINS to return values even if there are no matching values in other tables.

SELECT a.outlet_id, 
       a.outlet_name, 
       a.owner_name, 
       a.address, 
       a.pin, 
       a.outlet_place, 
       a.contact_no, 
       a.img, 
       a.status, 
       b.NAME, 
       b.phone, 
       b.id, 
       (SELECT Sum(total_amount) 
        FROM   tbl_orders 
        WHERE  status = "delivered" 
               AND outlet_id = a.outlet_id 
               AND a.status = "active") AS totalamount 
FROM   tbl_outlet AS a 
LEFT OUTER JOIN tbl_staff_to_outlet AS c ON a.outlet_id = c.outlet_id 
LEFT OUTER JOIN tbl_staff AS b ON b.id = c.staff_id AND b.status = "active" 
GROUP  BY a.outlet_id