Sessions That Never Ended — IS NULL in SQL

The problem

Streamhub's platform monitoring team is investigating a potential connection leak and needs to see all sessions still in progress.

Write a query to return the ID and start time of every open session.

Assumptions:

The sessions table contains every session ever recorded on the Streamhub platform.
The ended_at column records when each session closed; sessions still in progress have ended_at set to NULL.

Output:

One row per open session, with columns id and started_at.

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

Check answerShift Ctrl ↵

Run previews · Check grades

Write a query, then run it to see results here.

Worked solution Try it yourself first

Solution query

SELECT
  id,
  started_at
FROM
  sessions
WHERE
  ended_at IS NULL

The shape

"Still in progress" is encoded as the absence of an end time. Streamhub's session table has no boolean is_open flag — it has an ended_at column that gets filled in only when the session closes. IS NULL on that column is the open-session filter.

Clause by clause

SELECT id, started_at returns the two columns the monitoring team needs — which session, and when it started. ended_at stays out of the output; it's NULL for every row in the result.
FROM sessions reads the session log. The prompt's assumption that this table contains every session ever recorded is what makes this a complete scan, not a sample.
WHERE ended_at IS NULL keeps only rows where the end timestamp is absent. IS NULL returns true for in-progress sessions and false for closed ones.

The trap

WHERE ended_at = NULL returns zero rows on a connection-leak investigation, which is exactly when a false-empty result is most dangerous. The query reports "no open sessions" while the leak is still active. = against NULL evaluates to unknown, never true. IS NULL is the only operator that detects absence.

You practiced using a NULL filter to identify in-progress records. The 'still open / not yet finished' pattern recurs anywhere a status is captured by an end-timestamp column rather than a boolean flag.

Return the ID and start time of every open session

The shape

Clause by clause

The trap

Reading explains SQL. Writing it, over and over with instant feedback, is what makes you fluent.