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N023-M1 Tier 2 · Core SQL · medium analytics · Streamhub

Return the user IDs that appear in both the `sessions` table and the `conversions` table

Part of UNION, UNION ALL, INTERSECT, EXCEPT in SQL

The problem

Streamhub's conversion team wants to know which users have both started at least one session and made at least one conversion.

Write a query to return the user IDs that appear in both the sessions table and the conversions table.

Assumptions:

  • The sessions table records each session; user_id identifies the user.
  • The conversions table records each paid conversion; user_id identifies the user.
  • A user appears in the result exactly once, regardless of how many sessions or conversions they have.

Output:

  • One row per user who appears in both tables, with a single column user_id.
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
  user_id
FROM
  sessions
INTERSECT
SELECT
  user_id
FROM
  conversions

The shape

INTERSECT returns only rows that appear in both result sets, after deduplication. The output is the list of users who show up in sessions and also show up in conversions — the conversion team's session-plus-conversion overlap, computed directly as a set intersection.

Clause by clause

  • SELECT user_id FROM sessions is the left input. Every row in sessions projects its user. A user with many sessions contributes many rows here, but INTERSECT will deduplicate before comparing, so the multiplicity on each side does not change the output.
  • INTERSECT is the set operator. PostgreSQL evaluates each query independently, deduplicates each side, then returns rows that are present in both. The comparison is on the full row across both inputs — for a single-column query like this one, that means the user_id values themselves.
  • SELECT user_id FROM conversions is the right input. Same column, same type. A user with multiple conversions contributes multiple rows here too, but they collapse to one before the intersection runs.
  • The prompt asks for each qualifying user exactly once, which is the default shape INTERSECT produces. No extra step is needed to remove duplicates.

Why this and not UNION

UNION would combine the two queries and return every user who appears in either — anyone with a session, anyone with a conversion, deduplicated. That's the wrong question. The conversion team wants the and, not the or: users who are in sessions and in conversions simultaneously. UNION answers "appears in at least one"; INTERSECT answers "appears in both." The difference between the operators is the difference between the two questions, and picking the wrong one produces a much larger result set that looks plausible until someone checks the row count against either input.

You practiced INTERSECT for set-intersection logic. The recurring rule: INTERSECT returns rows present in both inputs after deduplication — equivalent to a JOIN on the matching columns followed by a DISTINCT, but more readable when set-membership is the question.

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