SQL Get all orders which contain exclusively one kind of item - mysql

Question

In this Database I want to count the number of orders that contain only products of a certain category.
I know how to count all orders that also contain items of a certain category, i.e. category 1:
SELECT Count(DISTINCT orderdetails.orderid) AS "AllCat1"
FROM orderdetails
INNER JOIN orders
ON orderdetails.orderid = orders.orderid
AND orderdetails.productid IN (SELECT DISTINCT productid
FROM products
WHERE categoryid = 1)
WHERE orderdate BETWEEN "1996-12-01" AND "1996-12-31";
I am having trouble finding an elegant way to get all orders that contain only category 1 items. I tried selecting all OrderIDs and grouping them by OrderID AND CategoryID:
SELECT *
FROM orderdetails
INNER JOIN orders
ON orderdetails.orderid = orders.orderid
AND orderdate BETWEEN "1996-12-01" AND "1996-12-31"
INNER JOIN products
ON orderdetails.productid = products.productid
GROUP BY orderdetails.orderid,
categoryid;
But I have no idea how to count all OrderIDs that contain category 1 items exclusively. Is my approach right? Or is there a better way to do it (Which I am sure there is)

Answer 1

You can use group by and having . . . but you need two levels. To get the orders that are all in one (or a set of categories) by doing:
SELECT o.orderId
FROM orders o JOIN
orderdetails od
ON od.orderid = o.orderid JOIN
products p
ON p.productid = od.productid
WHERE o.orderdate BETWEEN '1996-12-01' AND '1996-12-31'
GROUP BY o.orderId
HAVING SUM(CASE WHEN p.categoryid IN (1) THEN 1 ELSE 0 END) = COUNT(*);
The count needs a subquery:
SELECT COUNT(*)
FROM (SELECT o.orderId
FROM orders o JOIN
orderdetails od
ON od.orderid = o.orderid JOIN
products p
ON p.productid = od.productid
WHERE o.orderdate BETWEEN '1996-12-01' AND '1996-12-31'
GROUP BY o.orderId
HAVING SUM(CASE WHEN p.categoryid IN (1) THEN 1 ELSE 0 END) = COUNT(*)
) o;

Answer 2

You can do filtering using HAVING clause. We basically Count the order details rows where category is 1 for an order. It should be equal to the total count of rows for that order. This would ensure that all the categories in an order is 1 only.
SELECT od.orderid
FROM orderdetails AS od
INNER JOIN orders AS o
ON od.orderid = o.orderid
AND o.orderdate BETWEEN "1996-12-01" AND "1996-12-31"
INNER JOIN products AS p
ON od.productid = p.productid
GROUP BY od.orderid
HAVING COUNT(CASE WHEN p.categoryid = 1 THEN 1 END) = COUNT(*)
It is advisable to use Aliasing in case of multi-table queries for Code clarity and readability