MediumSQL challenge #180GROUP BYHAVING

Find Orders Containing More Than One Product

Return the orders that include at least two different products.

The challenge runs on the SQLabHub practice database (SQLite) and uses the order_items table. You can solve it in the browser, with nothing to install.

Solve This SQL Challenge

The task

Find orders containing more than one distinct product.

A correct answer returns 149 rows with 2 columns: order_id, COUNT(DISTINCT product_id).

Tables used in this challenge

order_items 496 rows

id INTEGER (PK) · order_id INTEGER · product_id INTEGER · quantity INTEGER · unit_price REAL · subtotal REAL

First 3 rows of the order_items table
idorder_idproduct_idquantityunit_pricesubtotal
11765192.21961.05
21823107.1321.3
316515.395.39

The first rows of each table are shown. See how all the tables relate on the database schema page.

Expected output

Expected output of the challenge
order_idCOUNT(DISTINCT product_id)
13
32
54
74
82

First 5 of 149 rows. This is the output of the model solution on the practice database. The task does not fix a sort order, so your rows may come back in a different order.

What you practise

  • Summarising many rows into one value with COUNT, SUM, AVG, MIN or MAX
  • Counting different values with COUNT(DISTINCT …)
  • Grouping rows with GROUP BY to get one result row per group
  • Filtering groups after aggregation with HAVING

Hint and approach

Try the challenge first. Open these only when you need them.

Show the hint

COUNT DISTINCT product_id

Show the step-by-step approach

How the model solution is built, in the order the database evaluates it:

  1. Read from the order_items table.
  2. Group the rows by order_id, so the result has one row for each value.
  3. For each group, compute COUNT(DISTINCT product_id), which counts each different value once.
  4. Filter the groups in HAVING: keep those where COUNT(DISTINCT product_id) is greater than 1.

Model solution

Checking your session…

What to watch for

An order with two lines for the same product has two rows but one distinct product, so it is excluded. That is the difference between HAVING COUNT(*) > 1 and HAVING COUNT(DISTINCT product_id) > 1.

  • HAVING. Conditions on single rows belong in WHERE and conditions on aggregates in HAVING. Putting a row filter in HAVING works, but it groups rows that are about to be thrown away.
  • COUNT(DISTINCT). After a join, COUNT(*) counts joined rows. Counting the parent entity needs COUNT(DISTINCT its id).
  • GROUP BY. Every selected column must either be in GROUP BY or sit inside an aggregate. SQLite accepts other columns and returns an arbitrary value for them; most databases raise an error.
Solve This SQL Challenge

Keep learning