Sql monthly report

Question

Hello. I have a problem that it may be trivial for you but gave me a lot of headaches. I got 4 tables. This is the code for all of them:
CREATE TABLE customers
     name           VARCHAR2(40) NOT NULL PRIMARY KEY,
     address      VARCHAR2(40) NOT NULL,
     balance          NUMBER(10) NOT NULL
INSERT INTO customers VALUES ('Shaun Turtle', '4 Family Avenue', '-200');
INSERT INTO customers VALUES ('Samantha Salamander', '53 Lois Lane', '-50');
INSERT INTO customers VALUES ('Ruth Raptor', '21 Stony Street', '43');
CREATE TABLE orders
     order_no     NUMBER(8) NOT NULL PRIMARY KEY,
     order_date     DATE NOT NULL,
     cust          VARCHAR2(40) NOT NULL REFERENCES customers(name)
INSERT INTO orders VALUES ('624', '10-Mar-2011', 'Shaun Turtle');
INSERT INTO orders VALUES ('625', '20-Mar-2011', 'Ruth Raptor');
INSERT INTO orders VALUES ('626', '25-Mar-2011', 'Shaun Turtle');
CREATE TABLE includes
     order_no     NUMBER(8) NOT NULL REFERENCES orders(order_no),
     item          VARCHAR2(40) NOT NULL ,
     qty          NUMBER(10) NOT NULL,
     PRIMARY KEY(order_no,item)
INSERT INTO includes VALUES ('624', 'Cheddar', '3');
INSERT INTO includes VALUES ('624', 'Chardonnay', '6');
INSERT INTO includes VALUES ('625', 'Cheddar', '5');
INSERT INTO includes VALUES ('625', 'Escargot', '12');
INSERT INTO includes VALUES ('625', 'Rocket', '1');
INSERT INTO includes VALUES ('625', 'Walnuts', '1024');
INSERT INTO includes VALUES ('626', 'Walnuts', '2048');
CREATE TABLE supplies
     name          VARCHAR2(40) NOT NULL,
     item          VARCHAR2(40)NOT NULL,
     price          NUMERIC(8,2) NOT NULL,
     PRIMARY KEY(name, item)
INSERT INTO supplies VALUES ('Acme', 'Cheddar', '3.49');
INSERT INTO supplies VALUES ('Acme', 'Chardonnay', '5.19');
INSERT INTO supplies VALUES ('Acme', 'Walnuts', '0.06');
INSERT INTO supplies VALUES ('Acme', 'Escargot', '0.25');
INSERT INTO supplies VALUES ('Ajax', 'Cheddar', '3.90');
INSERT INTO supplies VALUES ('Ajax', 'Chardonnay', '5.09');
INSERT INTO supplies VALUES ('Ajax', 'Rocket', '0.69');
The task is: At the end of the month the organisation likes to produce a report which details each customer’s real balance taking into account, firstly their current balance, and then all orders made during that month. They always ensure that each item ordered comes from the cheapest supplier. The report must include the following information: customer’s name, address, old balance, new balance and number of items that have been ordered by that customer during that period (you do not need to count the number of orders for any one customer). Your task is to produce the report for March 2011, assuming that current balances shown are correct as of the end of February 2011. Note that the report should be written using the minimal number of SQL statements possible.
Can you help me with this please?

Answer 1

It appears like a school homework, the tables don't have PKs and there are not normalized
The problem is also classic, I suggest you to apply divide and conquer strategy, you need to calculate the balance at the end of the last month, and the current month operations... so, you have 2 very different queries. Write one of the queries, then write the second, and finally try to put both togheter using "union all"
Also, you can found usefull to use an imaginary column like row_type with values 0 (for the balance) and 1 (for the last operations), and order the final query by row_type and date
I hope this help
Alfonso