Query returning summary for master categories

Question

Dear all,
i am using oracle database 11g r2.
i have the following 2 tables,
create tabel expenses_heads (exp_head_id number primary key,
                                             Main_exp_name varchar2(100) not null,
                                             refer_exp_head_id number);
alter table expenses_heads add constraint fk_refer_head_id foreign key(refer_exp_id) references expenses_heads(exp_head_id);
create table expenses (exp_id number primary key,
                                   exp_head_id number references  expenses_heads(exp_head_id) not null,
                                   exp_name varchar2(100) not null,
                                   exp_date date not null,
                                   exp_amount number not null,
                                   receipt varchar2(20),
                                   remarks varchar2(4000));
i have the following data,
insert into expenses_heads values(1,'School',null);
   insert into expenses_heads values(2,'College',null);
insert into expenses_heads values(3,'Bills',1);
insert into expenses_heads values(4,'Hardware',1);
  insert into expenses_heads values(5,'Food',1);
insert into expenses_heads values(6,'Dress',1);
insert into expenses_heads values(7,'Hardware',1);
  insert into expenses_heads values(8,'Admission',2);
   insert into expenses_heads values(9,'Office',2);
insert into expenses values(1,3,'Electricity Bill',sysdate,1000,null,null);
insert into expenses values(2,3,'Water Bill',sysdate,100,null,null);
insert into expenses values(3,4,'Chair',sysdate,1000,null,null);
  insert into expenses values(4,4,'Tables',sysdate,200,null,null);
  insert into expenses values(5,8,'Admin Ceremony',sysdate,3000,null,null);
    insert into expenses values(6,9,'Stationary',sysdate,2000,null,null);
commit;
now i want to write a query which have tow columns summary, Main_exp_name(from expenses_heads) and Total_amount(from expenses)
result should look like below
Main_exp_name                         Total_Amount
College                                        2300
university                                     5000
i think it could be written by using group by and rollup or cube or grouping sets, but confused.
kindly help.

Answer 1

    select refer_exp_id, 
      exp_head_id, 
      prior main_exp_name main_exp_name 
    from expenses_heads 
      connect by refer_exp_id = prior exp_head_id 
  h 
This is a Hierarchical Query to build the Hierarchy of your Expenses_Head Table. i.e. It will return you the Expenses Heads with its Parent Expense Details.
expenses e
join
   select refer_exp_id,
   exp_head_id,   
   prior main_exp_name main_exp_name
   from expenses_heads
   connect by refer_exp_id = prior exp_head_id
) h
on
( e.exp_head_id = h.exp_head_id )
You then Join this data with your Expenses table based on EXP_HEAD_ID.
select h.main_exp_name, sum (exp_amount) exp_amount
group by h.main_exp_name;
Group the data based on your Parent Expense i.e. MAIN_EXP_NAME, fetched from the Hierarchical Query.
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I hope it clears your questions.