需要产品清单和订单

Question

This is my homework task:

Products which were ordered along with the 5 most ordered products more than once and the count of orders they were included in. (Do not include the 5 most ordered products in the final result)

Products and orders are in same table. Order detail contain Order detail ID, order id, product id, quantity.

I've tried everything but I'm struggling with "along with" statement in the query.

Here is a query I have tried:

select 
  productid,
  count
  (
    (select productid from orderdetails)
     and
    (select productid from orderdetails order by quantity desc limit 5)
  ) as ORDERS
from orderdetails 
group by productid
order by ORDERS desc

Answer 1

You select from orderdetails, aggregate to get one result row per product and you count. It is very common to count rows with COUNT(*) , but you can also count expressions, eg COUNT(mycolumn) where you just count those that are not null. You are counting an expresssion (because it is not COUNT(*) but COUNT(something else) that you are using). The expression to test for null and count is

(select productid from orderdetails)
  and 
(select productid from orderdetails order by quantity desc limit 5)

This, however is not an expression that leads to one value that gets counted (when it's not null) or not (when it's null). You are selecting all product IDs from the orderdetails table and you are selecting all the five product IDs from the orderdetails table that got ordered with the highest quantity. And then you apply AND as if these were two booleans, but they are not, they are data sets. Apart from the inappropriate use of AND which is an operator on booleans and not on data sets, you are missing the point here that you should be looking for products in the same order, ie compare the order number somehow.

So all in all: This is completely wrong. Sorry to say that. However, the task is not at all easy in my opinion and in order to solve it, you should go slowly, step by step, to build your query.

Products which were ordered along with the 5 most ordered products more than once

Dammit; such a short sentence, but that is deceiving ;-) There is a lot to do for us...

First we must find the 5 products that got ordered most. That means sum up all sales and find the five top ones:

select productid
from orderdetails
group by productid
order by sum(quantity) desc
limit 5

(The problem with this: What if six products got ordered most, eg products A, B, and C with a quantity of 200 and products D, E, and F with a quantity of 100? We would get the top three plus two of the top 4 to 6. In standard SQL we would solve this with a ties clause, but MySQL's LIMIT doesn't feature this.)

Anyway. Now we are looking for products that got ordered with these five products along. Does this mean with all five at once? Probably not. We are rather looking for products that were in the same order with at least one of the top five.

with top_5_products as
     (query above)
   , orders_with_top_5 as
     (select orderid
      from orderdetails 
      where productid in (select productid from top_5_products)
     )
   , other_products_in_order as
     (select productid, orderid
      from orderdetails 
      where orderid in (select orderid from orders_with_top_5)
      and productid not in (select productid from top_5_products)

And once we've got there, we must even find products that got ordered with some of the top 5 "more than once" which I interpret as to appear in at least two orders containing top 5 products.

with <all the above>
select productid
from other_products_in_order
group by productid
having count(*) > 1;

And while we have counted how many orders the products share with top 5 products, we are still not there, because we are supposed to show the number of orders the products were included in, which I suppose refers to all orders, not only those containing top 5 products. That is another count, that we can get in the select clause for instance. The query then becomes:

with <all the above>
select
  productid,
  (select count(*) from orderdetails od where od.productid = opio.productid)
from other_products_in_order opio
group by productid
having count(*) > 1;

That's quite a lot for homework seeing that you are struggling with the syntax still. And we haven't even addressed that top-5-or-more ties problem yet (for which analytic functions come in handy).

The WITH clause is available since MySQL 8 and helps getting such a query that builds up step by step readable. Old MySQL versions don't support this. If working with an old version I suggest you upgrade :-) Else you can use subqueries directly instead.