I have a database and I need to create a query that will retrieve a list of customer names and the average order value made by each customer.
I tried:
SELECT c.customer_name, AVG(COUNT(o.order_id)*f.price) AS 'avgorderprice'
FROM Customers c
LEFT JOIN Orders o ON o.customer_id = c.customer_id
INNER JOIN F_in_Or fio ON o.order_id = fio.order_id
INNER JOIN Films f ON fio.film_id = f.film_id;
This is my database structure:
But I get an error, what can be wrong?
But I get an error, what can be wrong ?
You are trying to use an aggregate function Count within an aggregate function and getting a MISUSE OF AN AGGREGATE FUNCTION.
i.e. - > misuse of aggregate function COUNT()
Additionally you are not GROUP'ing your results and thus will receive the entire average rather than a per-customer average.
Aggregate functions work on a GROUP the default being all unless a GROUP BY clause places the rows into GROUPS.
You could instead of AVG(COUNT(o.order_id)*f.price) use sum(f.price) / count(*). However, there is an average aggregate function avg. So avg(f.price)' is simpler.
Additionally as you want an average per customer you want to use a GROUP BY c.customer clause.
Thus you could use :-
SELECT
c.customer_name,
avg(f.price) AS 'avgorderprice' --<<<<< CHANGED
FROM Customers c
LEFT JOIN Orders o ON o.customer_id = c.customer_id
INNER JOIN F_in_Or fio ON o.order_id = fio.order_id
INNER JOIN Films f ON fio.film_id = f.film_id
GROUP BY c.customer_name --<<<<< ADDED
;
This would result in something like :-
I think this should work
SELECT c.customer_name, AVG(f.price) AS 'avgorderprice' FROM Customers c LEFT JOIN Orders o ON o.customer_id = c.customer_id LEFT JOIN F_in_Or fio ON o.order_id = fio.order_id LEFT JOIN Films f ON fio.film_id = f.film_id;
Related
Sorry I don't know if there's proper name for 'incomplete join' but consider this sort of query, designed to return details of every sale:
SELECT a.id, a.productId, b.productDescription FROM Sales a, AdditionalProductData b WHERE a.productId = b.productId;
In a situation where AdditionalProductData doesn't guarantee to to have a row for every productId, but I want to return a result for every row in Sales, how can I modify my query to return either null or some default value e.g. "unknown" in such cases? I want to ensure sales of unregistered products are not omitted.
(It is a slightly contrived example, and indicates a problem in the DB, but those are outside scope of the question)
Use OUTER JOIN :
SELECT a.productId, b.productName
FROM Products b LEFT JOIN
Sales a
ON a.productId = b.id;
but I want to return a result for every row in Sales
Do the table swapping :
SELECT a.productId, COALESCE(b.productName, 'unknown') AS productName
FROM Sales a LEFT JOIN
Products b
ON a.productId = b.id;
Never use commas in the FROM clause. Always use proper, explicit, standard, readable JOIN syntax.
You want a left join:
SELECT p.id, p.productName
FROM Products p LEFT JOIN
Sales s
ON s.productId = p.id;
I am guessing you really want at least one row per product. It doesn't make sense that you would have sales for non-existent products -- at least in most databases.
The above doesn't make sense -- only selecting from one table. You probably want something like this:
SELECT p.id, p.productName, SUM(s.amount)
FROM Products p LEFT JOIN
Sales s
ON s.productId = p.id
GROUP BY p.id, p.productName;
EDIT:
If you really do want one row per sales, then you still want a LEFT JOIN just in the other order:
SELECT s.*, p.productName
FROM Sales s LEFT JOIN
Products p L
ON s.productId = p.id;
I was initially looking to see a breakdown of the total dollar business that each vendor has done (indirectly via the distributor) with each customer, where I'm trying not to use the Inner Join Syntax and used the Query below for this purpose:
select customers.cust_id, Vendors.vend_id, sum(quantity*item_price) as total_business from
(((Vendors left outer join Products
on Products.vend_id = Vendors.vend_id)
left outer join OrderItems --No inner joins allowed
on OrderItems.prod_id = Products.prod_id)
left outer join Orders
on Orders.order_num = OrderItems.order_num)
left outer join Customers
on Customers.cust_id = Orders.cust_id
where Customers.cust_id is not null -- THE ONLY DIFFERENCE BETWEEN QUERY1 AND QUERY2
group by Customers.cust_id, Vendors.vend_id
order by total_business
Now, I am trying to see the query output results for all vendor-customer combinations, including those combinations where there was no business transacted and am trying to write this via a single SQL Query. My teacher provided this solution, but I honestly cannot understand the logic at all, as I've never come across Sub-queries.
select
customers.cust_id,
Vendors.vend_id,
sum(OrderItems.quantity*orderitems.item_price)
from
(
customers
inner join
Vendors on 1 = 1
)
left outer join --synthetic product using joins
(
orders
join
orderitems on orders.order_num = OrderItems.order_num
join
Products on orderitems.prod_id = products.prod_id
) on
Vendors.vend_id = Products.vend_id and
customers.cust_id = orders.cust_id
group by customers.cust_id, vendors.vend_id
order by customers.cust_id
Thanks a lot
I would write this query as:
select c.cust_id, v.vend_id, coalesce(cv.total, 0)
fro Customers c cross join
Vendors v left outer join
(select o.cust_id, v.vend_id, sum(oi.quantity * oi.item_price) as total
from orders o join
orderitems oi
on o.order_num = oi.order_num join
Products p
on oi.prod_id = p.prod_id
group by o.cust_id, v.vend_id
) cv
on cv.vend_id = v.vend_id and
cv.cust_id = c.cust_id
order by c.cust_id;
The structure is quite similar. Both version start by creating a cross product between all customers and vendors. This creates all the rows in the output result set. Next, the aggregation needs to be calculated at this level. In the above query, this is done explicitly as a subquery which aggregates the values to the customer/vendor level. (In the original query, this is done in the outer query.)
The final step is joining these together.
Your teacher should be encouraging you to use table aliases, particularly table abbreviations. You should also be encouraged to use the proper join. So, although you can express a cross join as an inner join with on 1=1, a cross join is part of the SQL language, not a hack.
Similarly, parentheses in the from clause can make the logic harder to follow. Explicit subqueries are more easily read.
I'm trying to get only customers that ordered both a "Gas Range" and a "Washer". I'm getting Customers who ordered a "Gas Range" and not a "Washer" and customers with both. I need the customer that meets both conditions. I'm close but a little stuck. Below is the query that I have so far. Please let me know if you need more information.
My Tables - CUSTOMER(CUST_NUM, CUST_NAME), ORDER_LINE(ORDER_NUM, PART_NUM), ORDERS(ORDER_NUM, CUST_NUM), PART(PART_NUM, PART_DESCRIPTION)
SELECT C.CUST_NAME AS [Customer(s) that ordered a Gas Range and Washer]
FROM CUSTOMER C
INNER JOIN ORDERS O
ON C.CUST_NUM = O.CUST_NUM
INNER JOIN ORDER_LINE OL
ON O.ORDER_NUM = OL.ORDER_NUM
INNER JOIN PART P
ON OL.PART_NUM = P.PART_NUM
WHERE P.PART_DESCRIPTION IN ('GasRange','Washer')
GROUP BY C.CUST_NAME
try the following
SELECT C.CUST_NAME AS [Customer(s) that ordered a Gas Range and Washer]
FROM CUSTOMER C
INNER JOIN ORDERS O
ON C.CUST_NUM = O.CUST_NUM
INNER JOIN ORDER_LINE OL
ON O.ORDER_NUM = OL.ORDER_NUM
INNER JOIN PART P
ON OL.PART_NUM = P.PART_NUM
INNER JOIN ORDERS O2
ON C.CUST_NUM = O2.CUST_NUM
INNER JOIN ORDER_LINE OL2
ON O2.ORDER_NUM = OL2.ORDER_NUM
INNER JOIN PART P2
ON OL2.PART_NUM = P2.PART_NUM
WHERE P.PART_DESCRIPTION IN ('GasRange') and P2.PART_DESCRIPTION IN ('Washer')
GROUP BY C.CUST_NAME
EDIT: Had a further look and I'm afraid that this can't be simplified in any other way than using WITH and complicated aggregate functions, which I would say would be more complicated than this - I think the other solution suggested using WITH won't work - it joins incorrectly. You definitely can't remove order line, and you have to use the order twice as well - if it was used once, it will cover only when the customer ordered it within one order, which is not what you wanted ;)
Try this...
So basically you need to join your Parts table again to ensure the same customer ordered a "Gas Range" and a "Washer". An IN, like in your current query functions as an OR therefore you are not getting the expected result.
WITH CTE AS (
SELECT DISTINCT O.CUST_NUM FROM ORDERS O
INNER JOIN ORDER_LINE OL
ON O.ORDER_NUM = OL.ORDER_NUM
INNER JOIN PART P
ON OL.PART_NUM = P.PART_NUM
INNER JOIN PART P2
ON OL.PART_NUM = P2.PART_NUM
WHERE P.PART_DESCRIPTION IN ('GasRange')
AND P2.PART_DESCRIPTION IN ('Washer')
)
SELECT C.CUST_NAME AS [Customer(s) that ordered a Gas Range and Washer]
FROM CUSTOMER C
INNER JOIN CTE O
ON C.CUST_NUM = O.CUST_NUM
Here is how I contructed the step-by-step:
M1. create a sub-query that will return CustomerId and total invoiced for that customer
M2. A second subquery that will give a list of distinct ProductIDs (along with product SKUs) and CustomerIDs.
M3. The M1 and M2 subqueries will be joined to make association between customer totals and products (for the same CustomerId).
M4. The query M3 will be fed to the final query that will just find the top 5 products.
I'm stuck on creating the distinct ProductID and customerID because they would have to be in aggregate functions in order to make them distinct.
Attached is an image that is the erwin diagram which helps understand the system.
If you can help me with M1-M4, I will greatly appreciate it. I'm not a programmer by trade but a business analyst.
--M1--
select C.CustomerId, COUNT(I.InvoiceId) TotalNumInvoices
from Customer C
JOIN Invoice I ON (I.CustomerId = C.CustomerId)
group by C.CustomerId
--M2: Incomplete--
select P.ProductID, P.SKU, C.CustomerID
from Product P
JOIN InvoiceLine IL ON (IL.ProductId = P.ProductId)
JOIN Invoice I ON (IL.InvoiceId = I.InvoiceId)
JOIN Customer C ON (C.CustomerId = I.CustomerId)
you can also use the DISTINCT keyword your select clause in order to get unique values. Try this for m2:
select DISTINCT p.productID, p.sku, i.customerID
from invoice i INNER JOIN invoiceLine il
ON i.invoiceID = il.invoiceID
JOIN product p
ON il.productID = p.productID;
I'm trying to do something like:
SELECT c.id, c.name, COUNT(orders.id)
FROM customers c
JOIN orders o ON o.customerId = c.id
However, SQL will not allow the COUNT function. The error given at execution is that c.Id is not valid in the select list because it isn't in the group by clause or isn't aggregated.
I think I know the problem, COUNT just counts all the rows in the orders table. How can I make a count for each customer?
EDIT
Full query, but it's in dutch... This is what I tried:
select k.ID,
Naam,
Voornaam,
Adres,
Postcode,
Gemeente,
Land,
Emailadres,
Telefoonnummer,
count(*) over (partition by k.id) as 'Aantal bestellingen',
Kredietbedrag,
Gebruikersnaam,
k.LeverAdres,
k.LeverPostnummer,
k.LeverGemeente,
k.LeverLand
from klanten k
join bestellingen on bestellingen.klantId = k.id
No errors but no results either..
When using an aggregate function like that, you need to group by any columns that aren't aggregates:
SELECT c.id, c.name, COUNT(orders.id)
FROM customers c
JOIN orders o ON o.customerId = c.id
GROUP BY c.id, c.name
If you really want to be able to select all of the columns in Customers without specifying the names (please read this blog post in full for reasons to avoid this, and easy workarounds), then you can do this lazy shorthand instead:
;WITH o AS
(
SELECT CustomerID, CustomerCount = COUNT(*)
FROM dbo.Orders GROUP BY CustomerID
)
SELECT c.*, o.OrderCount
FROM dbo.Customers AS c
INNER JOIN dbo.Orders AS o
ON c.id = o.CustomerID;
EDIT for your real query
SELECT
k.ID,
k.Naam,
k.Voornaam,
k.Adres,
k.Postcode,
k.Gemeente,
k.Land,
k.Emailadres,
k.Telefoonnummer,
[Aantal bestellingen] = o.klantCount,
k.Kredietbedrag,
k.Gebruikersnaam,
k.LeverAdres,
k.LeverPostnummer,
k.LeverGemeente,
k.LeverLand
FROM klanten AS k
INNER JOIN
(
SELECT klantId, klanCount = COUNT(*)
FROM dbo.bestellingen
GROUP BY klantId
) AS o
ON k.id = o.klantId;
I think this solution is much cleaner than grouping by all of the columns. Grouping on the orders table first and then joining once to each customer row is likely to be much more efficient than joining first and then grouping.
The following will count the orders per customer without the need to group the overall query by customer.id. But this also means that for customers with more than one order, that count will repeated for each order.
SELECT c.id, c.name, COUNT(orders.id) over (partition by c.id)
FROM customers c
JOIN orders ON o.customerId = c.id