MediumSQL challenge #187HAVINGSUBQUERY

Users Above Avg Reviews

Find users whose review count is above average.

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

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The task

Find users whose review count is above average.

A correct answer returns 49 rows with 2 columns: user_id, cnt.

Tables used in this challenge

reviews 180 rows

id INTEGER (PK) · product_id INTEGER · user_id INTEGER · rating INTEGER · comment TEXT · reviewed_at TEXT · helpful_votes INTEGER

First 3 rows of the reviews table
idproduct_iduser_idratingcommentreviewed_athelpful_votes
123455Works perfectly. Very happy with this purchase.2022-09-0240
2101685Excellent build quality. Will buy again!2023-01-0596
340391Would give 6 stars if I could!2023-05-1517

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
user_idcnt
33
102
163
173
202

First 5 of 49 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
  • Grouping rows with GROUP BY to get one result row per group
  • Filtering groups after aggregation with HAVING
  • Using the result of one query inside another
  • Querying the result of another query with a subquery in FROM

Hint and approach

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

Show the hint

Subquery with GROUP BY

Show the step-by-step approach

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

  1. Read from the reviews table.
  2. Group the rows by user_id, so the result has one row for each value.
  3. For each group, compute COUNT(*).
  4. Filter the groups in HAVING: keep those where cnt is greater than the value returned by a subquery.
  5. The subquery in HAVING runs once and returns a single value. It works like this:
    1. Build an inner query first (a derived table):
      1. Read from the reviews table.
      2. Group the rows by user_id, so the result has one row for each value.
      3. For each group, compute COUNT(*).
    2. Aggregate over all the rows that are left: compute AVG(review_count). With no GROUP BY, the result is a single row.

Model solution

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What to watch for

  • Derived table. Columns computed in the inner query are only visible outside under their alias, so every calculated column there needs a name.
  • 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.
  • 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.
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