Working with multiple tables - mysql

Question

I have these 4 tables with different rows and columns and I need to get certain values. I am having some difficulty with a few and cannot seem to get it.
customers table:
customerID: integer
fName: string
lName: string
items table:
itemID: integer
description: string
price: float
orders table:
orderID: integer
itemID: integer
aID: integer
customerID: integer
date: date
addresses table:
aID: integer
housenum: integer
streetName: string
town:string
state: string
zip:integer
The values I am looking to get are:
List the itemID and description of all items for which there is only one order.
Now I can get the itemID and description, however I am not sure how to nail it down to items only ordered once. I am thinking of using JOIN and GROUP BY but I cannot seem to find the correct combination.
SELECT itemID,
description
FROM items
GROUP BY itemID
List all customers who have shipped items to multiple addresses.
This one I have to use JOIN somehow, since the customerID would be the same but not the address?
SELECT customerID,
streetName,
housenum
FROM addresses
JOIN customers
ON addresses.streetName = customers.customerID
Return the total revenue generated by all of the orders.
Couldn't I just use:
SELECT SUM(price),
orderID
FROM items
JOIN orders
ON orders.orderID = items.price ?
List the first and last names of all customers who have had the same item shipped to at least two different addresses.
SELECT fName,
lName,
itemID,
streetName,
housenum
FROM addresses,
customers,
items,
somehow join multiple(2+)?

Answer 1

I may not be right with all the queries but I have found the items listed on the question very interesting and decided to give it a try.
items sold only once
SELECT o.itemID,
i.description
FROM orders o
LEFT JOIN items i
ON i.id = o.itemID
GROUP BY o.itemID
HAVING COUNT(*) = 1;
On this query, you need to join the the items table in order to retrieve the description and GROUP BY it by the itemID so you can specifically use COUNT for each itemID which we can then use with HAVING to obtain on the ones with 1 order.
Customer that have shipped to diff address
SELECT COUNT(DISTINCT addressID) as diff_address,
c.fName,
c.lName
FROM orders o
LEFT JOIN customers c
ON o.customerID = c.id
GROUP BY o.customerID
HAVING diff_address > 1;
Here I am using the DISTINCT function to eliminate duplicated address and GROUP BY the customerID so I can have a list of unique address per customer and again using HAVING we get the customers that have shipped products with more than 1 different address. Am also using JOIN on the customer table to obtain their needed information.
Total revenue
SELECT SUM(i.price) as total
FROM orders o
LEFT JOIN items i
ON o.itemID = i.id;
I guess this one is the simplest one here, by using JOIN on the items table, I am able to read each item price which I can then sum per order to have a gran total of sales.
List of customers that have shipped the same item to at least 2 different address
SELECT c.fName,
c.lName,
COUNT(oi.itemID) AS `total items with diff address >= 2`
FROM customers c
JOIN (SELECT customerID,
itemID,
COUNT(DISTINCT addressID) AS diff_address
FROM orders
GROUP BY itemID, customerID
HAVING diff_address >= 2
) AS oi
ON oi.customerID = c.id
GROUP BY c.id;
This one was the most complicated one, hope I can explain it. First I started it with the sub-query where I can list all items per customer that have multiple address, then by using JOIN on the customerID I can count how many different items that given customer have shipped with 2 or more diff address.
Here is a LIVE DEMO.
If you see any of the queries are not good or have any advice kindly do tell me so I can improve it.