SQL查询从多个表返回数据？

Question

I have tables with the following schemas:

items (itemID: integer, description: string, price: integer)
orders (orderID: integer, itemID: integer, aID: integer, customerID: integer, date: date)

I would like to find out each item for which there is only one order, return the itemID, description, and the date of the order.

I have the following code so far:

SELECT *
FROM (
SELECT *
FROM orders
GROUP BY itemID
HAVING COUNT(*) = 1
) AS ONLY_ONCE

But this returns only the information from the orders table (orderID, itemID, aID, and customerID) in my code I search for items for which the itemID appears only once in the orders table, which means that it was only ordered once. How do I get the description of these items which is in the items table? I tried using the join function to do this but was not successful.

Thanks

Answer 1

You were very close... try this:

SELECT *
FROM items
WHERE itemID IN (
    SELECT itemID
    FROM orders
    GROUP BY itemID
    HAVING COUNT(*) = 1
) 

If you also want the date from orders in your result set, there are many ways to do that, but I'd opt for a CTE and an inner join over the IN operator, like so:

;WITH SingleOrders AS (
    SELECT itemID, MAX([date]) AS [Date]
    FROM orders
    GROUP BY itemID
    HAVING COUNT(*) = 1
) SELECT *
    FROM items i
         INNER JOIN SingleOrders so ON i.itemID = so.itemID

The first part declares a common table expression called "SingleOrders" which consists of two columns: the ID and the Date for each itemID that has only one order. The MAX(date) returns the one and only date, and is required because of the GROUP BY clause.

The actual SELECT statement then joins this CTE with the items table to select out only the itemIDs that have just one order, and since it includes all the columns from both the items and the SingleOrders tables, will include the date.

If you don't want to use a CTE (or can't use one), something like this might work for you... just add an inner join back to the orders table to pick up the date:

SELECT i.*, o.date
FROM items i
     INNER JOIN orders o ON i.itemID = o.itemID
WHERE i.itemID IN (
    SELECT itemID
    FROM orders
    GROUP BY itemID
    HAVING COUNT(*) = 1
) 

Answer 2

You need to join your 'group' table result with the original data

SELECT I.*
FROM items I
INNER JOIN(
    SELECT itemID
    FROM orders
    GROUP BY itemID HAVING COUNT(1)=1
    ) O ON O.itemID=I.itemID