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N039-M4 Tier 3 · Intermediate · medium analytics · Streamhub

Return the ID and event count of every session, plus the session's dense rank by `event_count` in descending order

Part of ROW_NUMBER, RANK, DENSE_RANK in SQL

The problem

Streamhub's engagement analyst ranks sessions by their event count — sessions with the same event count share a rank, with no positions skipped.

Write a query to return the ID and event count of every session, plus the session's dense rank by event_count in descending order.

Assumptions:

  • The sessions table has one row per session with an id and an event_count.
  • Rank 1 goes to sessions with the highest event_count, with rank increasing as event_count decreases.
  • Sessions with the same event_count share a rank, and the next rank is always exactly one higher.

Output:

  • One row per session, with columns id, event_count, and engagement_rank.
Schema · analytics 5 tables
users
id integer
name text
email text
country text
plan text
signed_up_at timestamptz
is_active boolean
conversions
id integer
user_id integer
converted_at timestamptz
plan text
amount numeric
sessions
id integer
user_id integer
started_at timestamptz
ended_at? timestamptz
event_count integer
events
id integer
user_id integer
session_id? integer
event_type text
occurred_at timestamptz
properties? jsonb
periods
id integer
name text
start_month integer
end_month integer
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Solution query
SELECT
  id,
  event_count,
  DENSE_RANK() OVER (
    ORDER BY
      event_count DESC
  ) AS engagement_rank
FROM
  sessions

The shape

DENSE_RANK() OVER (ORDER BY event_count DESC) groups sessions into consecutive engagement tiers: every session that shares an event_count value gets the same tier number, and the next-distinct count gets the next consecutive integer. The output reads as a tier map, not a competitive ranking.

Clause by clause

  • SELECT id, event_count, DENSE_RANK() OVER (ORDER BY event_count DESC) AS engagement_rank returns each session's ID, its event count, and its engagement tier. The window's ORDER BY event_count DESC sorts sessions from highest activity to lowest; without PARTITION BY, the entire sessions table is one window. DENSE_RANK then walks the sorted sequence assigning 1 to the highest count, 2 to the next-distinct count, and so on.
  • FROM sessions reads every session. No filter; every session is placed in a tier.

Why DENSE_RANK and not RANK

The brief asks for engagement tiers, which is a categorical reading of the data. DENSE_RANK returns 1, 2, 3, ... consecutively across distinct values, so tier 1 is the highest engagement, tier 2 is the next-highest, and the integer is directly interpretable as "which band does this session sit in." RANK would skip ahead after ties, leaving gaps that mean nothing in a tier-counting context. When the reader is meant to count tiers ("there are five engagement bands"), DENSE_RANK is the correct function.

The trap

A session showing engagement_rank = 1 is not necessarily the single most active session; it shares its tier with every other session at the same event_count. DENSE_RANK deliberately collapses ties. If the question shifts to "which is the single most active session," DENSE_RANK is the wrong tool because it cannot distinguish ties at the top. The right answer there is ORDER BY event_count DESC LIMIT 1 or a tie-breaking secondary sort, not a ranking function.

You practiced DENSE_RANK() OVER (ORDER BY ... DESC) — gap-free ranking ordered descending; consecutive values stay continuous through ties.

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