So I am working on an exercise to improve my SQL skills and I need to modify my query to add a subquery that looks for the existence of a customer ID in the CUSTOMERS table. The exercise also notes that this is a correlated sub-query so you will have to match the customer ID from the sub-query to the customer ID in the outer query. The title of the section of this exercise is "Two Table Join with Subquery". After researching and trying for a few hours now, I've exhausted all my resources but one. Any help would be much appreciated! (I am using Oracle Apex for this.)
SELECT ORDER_ID, ORDER_MODE, CUSTOMER_ID, PRODUCT_ID
FROM ORDERS
NATURAL JOIN ORDER_ITEMS;
Using exists you can check for existence,
SELECT O.ORDER_ID, O.ORDER_MODE, O.CUSTOMER_ID, OI.PRODUCT_ID
FROM ORDERS O
INNER JOIN ORDER_ITEMS OI on O.ORDER_ID=OI.ORDER_ID
WHERE EXISTS(
SELECT * FROM CUSTOMERS C
WHERE O.CUSTOMER_ID=C.CUSTOMER_ID
);
This would do the second,
SELECT O.ORDER_ID, O.ORDER_MODE, O.CUSTOMER_ID, OI.PRODUCT_ID,P.TRANSLATED_NAME
FROM ORDERS O
INNER JOIN ORDER_ITEMS OI on O.ORDER_ID=OI.ORDER_ID
INNER JOIN PRODUCT_DESCRIPTIONS P on P.PRODUCT_ID=OI.PRODUCT_ID
WHERE EXISTS(
SELECT * FROM CUSTOMERS C
WHERE O.CUSTOMER_ID=C.CUSTOMER_ID
);
Related
I have the following tables:
Orders (OID, Count, ProductID, TableNr)
Table (TableNr, Name, Number)
Products (ProductID, Prize)
Now I want to calculate how much was earned per table. I think to do this I have to Group By Orders.TableNr. But how can I multiply the Products.Prize with the Orders.Count and after that sum up this results within the group?
You are describing a join and aggregation:
select o.tableNr, sum(o.count * p.prize) totalEarned
from orders o
inner join products p on p.productId = o.productId
group by o.tableNr
If you want to display table information as well (say, the table name), then you can add another join:
select t.tableNr, t.name, sum(o.count * p.prize) totalEarned
from table t
inner join orders o on o.tableNr = t.tableNr
inner join products p on p.productId = o.productId
group by t.tableNr, t.name
Note that table is a SQL keyword, hence not a good choice for a table name.
Hello I am trying to add a Translated_Name column from the Product_descriptions table to my current query that is already joining two tables however the translated_name column is a type NVARCHAR2. Should I be using Inner Join for it or am I completely wrong?
select order_mode,customer_id,product_id from ORDERS
inner join ORDER_items on order_items.ORDER_ID=Orders.ORDER_ID
where exists(select customer_id from customers where orders.customer_id=customers.customer_id)
inner join product_descriptions on product_descriptions.translated_name = Orders.Customer_id
The where clause goes after the joins:
select
order_mode,
customer_id,
product_id
from orders o
inner join order_items oi
on oi.order_id = o.order_id
inner join product_descriptions pd
on pd.translated_name = o.customer_id
where exists(
select 1
from customers c
where o.customer_id = c.customer_id
)
Notes:
table aliases make the query easier to read and write
you should qualify the columns the the from clause with the alias of the table they belong to
I am quite suspicious about the join condition on product_descriptions, which involves customer_id; you might need to review that (without knowing your table structures, it is not possible to tell what the correct condition is)
I'm having trouble with a really simple left join statement that's driving me nuts
I wanted to count the numbers of orders from each customer, that's fine, but I want to display the name, and I'm joining with the customers table and trying to select the name and it says that CustomerName is not part of an aggregate function, it's really weird.
SELECT Customers.CustomerName as 'Name',
COUNT(*) AS 'Order Count'
FROM Orders
LEFT JOIN Customers
ON Orders.CustomerID = Customers.CustomerID
GROUP BY Customers.CustomerID
Thanks for any tips.
You need to count the rows from the orders table, and the left join should be in the other direction:
SELECT c.customerid,
c.CustomerName as "Name",
COUNT(o.customerid) AS "Order Count"
FROM Customers c
LEFT JOIN Orders o ON o.CustomerID = cs.CustomerID
GROUP BY c.CustomerID, c.customername;
count() will ignore NULL values that come into the result due to the outer join so it will count the number of orders for each customers. Customers without orders will be show with a zero count.
Include CustomerName in Group BY instead of CustomerID
SELECT Customers.CustomerName as 'Name', COUNT(*) AS 'Order Count'
FROM Orders LEFT JOIN Customers ON Orders.CustomerID = Customers.CustomerID
GROUP BY Customers.CustomerName
If you are using SQL Server then try using OVER() without Group BY
SELECT Customers.CustomerName as 'Name', COUNT(*) OVER (PARTITION BY Customers.CustomerName ORDER BY Customers.CustomerName)AS 'Order Count'
FROM Orders LEFT JOIN Customers ON Orders.CustomerID = Customers.CustomerID
Modify as below. column used in group by clause should be in column queried in select clause
SELECT Customers.CustomerName as 'Name',
COUNT(*) AS 'Order Count'
FROM Orders
LEFT JOIN Customers
ON Orders.CustomerID = Customers.CustomerID
GROUP BY Customers.CustomerName
I have just reordered your query,please try this it will definitely work for you.
SELECT Customers.CustomerName as 'Name',
COUNT(*) AS 'Order Count'
FROM Customers
LEFT JOIN Orders
ON Customers.CustomerID=Orders.CustomerID
GROUP BY Customers.CustomerID
A simple approach to get all the columns in the customers table is to use a correlated subquery:
select c.*, -- or whatever columns you want
(select count(*)
from orders o
where o.CustomerID = c.CustomerID
) as order_count
from customers c;
Because this avoids the outer GROUP BY, this also has the advantage of having better performance in most databases, particularly with an index on orders(CustomerId). Plus, it returns 0 if the customer has no orders. And, it allows you to choose any or all of the columns from Customers.
The correct way to get the counts you want is to count a column from Orders:
SELECT c.CustomerName, c.CustomerID,
COUNT(o.CustomerId) AS Order_Count
FROM Customers c LEFT JOIN
Orders o
ON o.CustomerID = c.CustomerID
GROUP BY c.CustomerID, c.CustomerName;
Notes:
The Customers table goes first in the LEFT JOIN because presumably you want all rows in Customers.
Table aliases make the query easier to write and to read.
Do not use single quotes for column aliases, even if your database supports it. The best method is to choose aliases that do not need to be supported.
Include the CustomerId in the logic, just in case two customers have the same name.
Count a column from Orders so you get a count of 0 for customers with no orders.
Here is a simple query:
SELECT orders.id, customers.name, COUNT(order_product.id)
FROM orders
INNER JOIN order_product ON orders.id = order_product.order_id
INNER JOIN customers ON orders.customer_id = customers.id
GROUP BY orders.id;
In other words, I want:
The ID of an order.
The number of products (count) in each order.
The customer name of the order.
The problem is about selecting customers.name. I cannot select it directly because it's not in aggregate function nor group by. But there is only one, so I d'ont know why I have to aggregate it. I can do a trick like this to select its name:
SELECT MAX(customers.name)
But I think it's dirty, because I don't want the "max name of a customer for an order" but "the name of the customer for an order". What is the elegant way to do such a thing?
Hope it's clear and not a duplicate.
EDIT: an order have only one customer identified by orders.customer_id. That's why I asking why I have to do such a trick.
Add customers.name to the GROUP BY clause:
SELECT orders.id, customers.name, COUNT(order_product.id)
FROM orders
INNER JOIN order_product ON orders.id = order_product.order_id
INNER JOIN customers ON orders.customer_id = customers.id
GROUP BY orders.id, customers.name
Usually you can simply group by all selected columns that are not arguments to set functions!
Alternatively, you could use window functions
SELECT DISTINCT orders.id, customers.name, COUNT(order_product.id) OVER ( PARTITION BY orders.id)
FROM orders
INNER JOIN products ON orders.id = order_product.order_id
INNER JOIN customers ON orders.customer_id = customers.id;
The following statement works properly but shows each record 4 times. Repeated; I know the relationship is wrong but no idea how to fix it? Apologies if this is simple and i've missed it.
SELECT Customers.First_Name, Customers.Last_Name, Plants.Common_Name, Plants.Flower_Colour, Plants.Flowering_Season, Staff.First_Name, Staff.Last_Name
FROM Customers, Plants, Orders, Staff
INNER JOIN Orders AS t2 ON t2.Order_ID = Staff.Order_ID
WHERE Orders.Order_Date
BETWEEN '2011/01/01'
AND '2013/03/01'
You are generating a Cartesian product between the tables since you have not provided join syntax between any of the tables:
SELECT c.First_Name, c.Last_Name,
p.Common_Name, p.Flower_Colour, p.Flowering_Season,
s.First_Name, s.Last_Name
FROM Customers c
INNER JOIN Orders o
on c.customerId = o.customer_id
INNER JOIN Plants p
on o.plant_id = p.plant_id
INNER JOIN Staff s
ON o.Order_ID = s.Order_ID
WHERE o.Order_Date BETWEEN '2011/01/01' AND '2013/03/01'
Note: I am guessing on column names for the joins
Here is a great visual explanation of joins that can help in learning the correct syntax
In the FROM... clause you are doing a cross join - combining every customer with every plant with every order with every staff.
You should only mention one table in the FROM clause and then connect the other ones with INNER JOINS to only get related records.
I don't know exactly how your database looks like, but something like this:
SELECT Customers.First_Name, Customers.Last_Name, Plants.Common_Name,
Plants.Flower_Colour, Plants.Flowering_Season, Staff.First_Name, Staff.Last_Name
FROM Customers
INNER JOIN Orders ON Orders.Customer_ID = Customers.Customer_ID
INNER JOIN Staff ON Staff.Staff_ID = Orders.Staff_ID
INNER JOIN Plants ON Plants.Plants_ID = Orders.Plants_ID
WHERE Orders.Order_Date
BETWEEN '2011/01/01'
AND '2013/03/01'
This is because you are selecting from four tables without any joins between them, and also because you are joining Orders twice. As the result, a Cartesian product is made.
Here is how you should fix it: re-write the theta join using the ANSI syntax, and provide proper join conditions:
SELECT Customers.First_Name, Customers.Last_Name, Plants.Common_Name, Plants.Flower_Colour, Plants.Flowering_Season, Staff.First_Name, Staff.Last_Name
FROM Customers
JOIN Plants ON ...
JOIN Orders ON ...
JOIN Staff ON ...
INNER JOIN Orders AS t2 ON t2.Order_ID = Staff.Order_ID
WHERE Orders.Order_Date BETWEEN '2011/01/01' AND '2013/03/01'
Replace ... with proper join conditions; this should make the results look as expected.