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N029-E1 Tier 3 · Intermediate · easy analytics · Streamhub

Return the total number of sessions and the number of completed sessions as a single row

Part of NULL Handling in Joins and Aggregates in SQL

The problem

Streamhub's engineering team is auditing session data quality.

Write a query to return the total number of sessions and the number of completed sessions as a single row.

Assumptions:

  • The sessions table has one row per session with an ended_at value.
  • Sessions still in progress have a missing ended_at; completed sessions have a recorded ended_at.
  • The total number of sessions covers every session record. The number of completed sessions covers only sessions with a recorded ended_at.

Output:

  • A single row with columns total_sessions and completed_sessions.
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
  COUNT(*) AS total_sessions,
  COUNT(ended_at) AS completed_sessions
FROM
  sessions

The shape

COUNT(*) counts every session record, and COUNT(ended_at) counts only the records whose ended_at is recorded. Running them side by side in the same SELECT returns the two figures the audit needs from a single pass over sessions.

Clause by clause

  • SELECT COUNT(*) AS total_sessions, COUNT(ended_at) AS completed_sessions returns the two figures as a single row. COUNT(*) counts rows and ignores nullability entirely, so it returns the full 180 session records. COUNT(ended_at) only counts rows where ended_at is non-missing, so the in-progress sessions (whose ended_at is missing) are skipped and the result is 168 completed.
  • FROM sessions reads every session record. There is no WHERE because the audit covers the full population; the two aggregates do the work of separating completed from total inside the SELECT list itself.

Why this and not COUNT(ended_at) for both

COUNT(ended_at) would silently undercount the total, because the in-progress sessions have a missing ended_at and COUNT(column) skips missing values by design. The total figure has to be a count of rows, not of recorded values. COUNT(*) is the only form that ignores column nullability and returns the row count outright.

You practiced COUNT(*) vs COUNT(column) — the first counts every record, the second counts only records where the column is recorded.

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