SQL NOT IN with a Subquery
The mirror image of IN — and the one place a single NULL can break your whole query.
What & Why
NOT IN (subquery) keeps an outer value only when it differs from every known value returned by the one-column subquery. It is concise for anti-membership, but nullable subquery values must be removed or the predicate can become UNKNOWN for every candidate.
See How It Works
BUSINESS QUESTION
Marketing wants campaigns whose IDs never appear on a lead.
| id | name | channel | spend | start_date | end_date | status | target_segment |
|---|---|---|---|---|---|---|---|
| 1 | Spring Launch | google_ads | 55000.00 | 2024-01-15 | 2024-03-31 | active | smb |
| 2 | Retention Webinar | 45000.00 | 2024-02-10 | 2024-04-15 | active | enterprise | |
| 3 | Finance Retargeting | 50000.00 | 2024-03-12 | 2024-05-31 | active | enterprise | |
| 4 | Enterprise Search | google_ads | 60000.00 | 2024-04-01 | 2024-06-30 | active | enterprise |
| id | campaign_id | created_at | qualified_at | converted_at | lead_score | source | country | |
|---|---|---|---|---|---|---|---|---|
| 301 | 1 | ana@example.com | 2024-01-21 09:10:00+00 | 2024-01-22 11:00:00+00 | 2024-02-02 10:00:00+00 | 86 | google_ads | US |
| 302 | 1 | ben@example.com | 2024-01-24 12:40:00+00 | NULL | NULL | 52 | google_ads | CA |
| 303 | 2 | chloe@example.com | 2024-02-16 08:30:00+00 | 2024-02-18 14:20:00+00 | NULL | 74 | GB | |
| 304 | 3 | dev@example.com | 2024-03-20 17:15:00+00 | 2024-03-21 09:00:00+00 | 2024-04-04 16:00:00+00 | 91 | US |
EXAMPLE QUERY
SELECT
c.id AS campaign_id,
c.name
FROM marketing.campaigns c
WHERE c.id NOT IN (
SELECT l.campaign_id
FROM marketing.leads l
WHERE l.campaign_id IS NOT NULL
)
ORDER BY campaign_id;RESULT — campaign IDs absent from leads
| campaign_id | name |
|---|---|
| 4 | Enterprise Search |
The lead table contains campaign IDs 1, 2, and 3, leaving campaign 4.
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