How to get customer's items purchased? - sql

Question

I have two tables:
customer(customer_id (PK), first_name, last_name, phone)
orders(order_id (PK), item_name, quantity)
With customer_id = order_id (one order_id per customer, that identifies the customer).
Inside my database I have:
INSERT INTO customer(first_name, last_name, phone) VALUES ('Colin', 'Farell', '123453');
INSERT INTO customer(first_name, last_name, phone) VALUES ('Aaron', 'Smith', '123451');
INSERT INTO customer(first_name, last_name, phone) VALUES ('Becky', 'Roberts', '123452');
INSERT INTO orders(item_name, quantity) VALUES ('Tissues', 2);
INSERT INTO orders(item_name, quantity) VALUES ('Lamp', 1);
INSERT INTO orders(item_name, quantity) VALUES ('Chocolate', 3);
How can I get the list of items a specific customer bought? For instance, Colin bought 2 tissues, a lamp and 3 chocolates.
The right output would be (without the first_name repeating):
First_name - Item_name - Quantity
Colin - Tissues - 2
Colin - Lamp - 1
Colin - Chocolate - 3
I tried:
SELECT customer.first_name, customer.last_name, orders.item_name, orders.quantity
FROM customer, orders
WHERE customer.customer_id = orders.order_id
ORDER BY customer.first_name;
But it shows all the customers, not a specific one...
Is the problem also lying in: WHERE customer.customer_id = orders.order_id ? Because if a customer can have many orders, if I insert more things inside the orders table, then my WHERE clause would not make sense?
Any ideas to clear my confusion? thanks

Answer 1

Data model is wrong. Should be something like this:
SQL> create table customer
2 (customer_id number primary key,
3 first_name varchar2(20),
4 last_name varchar2(20),
5 phone varchar2(20));
Table created.
SQL> create table items
2 (item_id number primary key,
3 item_name varchar2(20));
Table created.
SQL> create table orders
2 (order_id number primary key,
3 customer_id number constraint fk_ord_cust references customer (customer_id)
4 );
Table created.
SQL> create table order_details
2 (order_det_id number primary key,
3 order_id number constraint fk_orddet_ord references orders (order_id),
4 item_id number constraint fk_orddet_itm references items (item_id),
5 amount number
6 );
Table created.
Some quick & dirty sample data:
SQL> insert all
2 into customer values (100, 'Little', 'Foot', '00385xxxyyy')
3 into items values (1, 'Apple')
4 into items values (2, 'Milk')
5 into orders values (55, 100)
6 into order_details values (1000, 55, 1, 5) -- I'm ordering 5 apples
7 into order_details values (1001, 55, 2, 2) -- and 2 milks
8 select * from dual;
6 rows created.
SQL> select c.first_name, sum(d.amount) count_of_items
2 from customer c join orders o on o.customer_id = c.customer_id
3 join order_details d on d.order_id = o.order_id
4 group by c.first_name;
FIRST_NAME COUNT_OF_ITEMS
-------------------- --------------
Little 7
SQL>
Or, list of items:
SQL> select c.first_name, i.item_name, d.amount
2 from customer c join orders o on o.customer_id = c.customer_id
3 join order_details d on d.order_id = o.order_id
4 join items i on i.item_id = d.item_id;
FIRST_NAME ITEM_NAME AMOUNT
-------------------- -------------------- ----------
Little Apple 5
Little Milk 2
SQL>

Answer 2

There is no relation between the two tables which you wish to combine data from. Kindly create a foreign key relation between the two tables which would help you get a common value based on which you could extract data.
For e.g. - The column Customer_id from customers table could be the foreign key in table orders which would specify the order placed by each customer.
The following query should return you the expected result:
SELECT customer.first_name, customer.last_name, orders.item_name, orders.quantity
FROM customer, orders
WHERE customer.customer_id = orders.customer_id
ORDER BY customer.first_name;
The query specified by you does not return any result as there is no match for any order and customer id in the two tables as both depict two different values.
Hope it helps. Cheers!

Answer 3

In your last query you have shown that your tables are linked ( customer.customer_id = orders.order_id ), but in the tables you have created, there is no link between them. I think this should work:
Step 1: Create a Customer table as follow:
Create table customer
(customer_id id primary key,
first_name varchar(25),
last_name varchar(25),
phone int);
Step 2: Create Items table as follow:
Create table Items
(item_id primary key,
item_name varchar(25));
Step 3: Create link table that relates two tables above as follow:
Create table Orders
(Customer_Id int,
Item_ID int,
Quantity int);
Step 4: Use this query to pull out the information you want:
select c.first_name,i.item_name,o.Quantity from
customer c inner join orders o on c.customer_id = o.customer_id
inner join items i on i.item_id = o.Item_id;
Please try it and let me know if there is any problem.