SQL NOT EXISTS
The safe way to ask 'which rows have no match' — unlike NOT IN, NULLs can't break it.
What & Why
NOT EXISTS keeps rows where the correlated subquery finds nothing. Because it checks row existence rather than comparing values in a list, it's immune to the NULL trap that makes NOT IN dangerous.
See How It Works
BUSINESS QUESTION
Marketing wants campaigns that have not produced any leads.
| 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 |
Watch NOT EXISTS keep the campaign with no leadStep 1 of 4
NOT EXISTS lead where lead.campaign_id = campaign.id
| campaign_id | campaign | lead ids | decision |
|---|---|---|---|
| 1 | Spring Launch | 301, 302 | SKIP |
| 2 | Retention Webinar | 303 | SKIP |
| 3 | Finance Retargeting | 304 | SKIP |
| 4 | Enterprise Search | — | KEEP |
Related leads make NOT EXISTS false for campaign 1.
EXAMPLE QUERY
SELECT
c.id AS campaign_id,
c.name,
c.channel
FROM marketing.campaigns c
WHERE NOT EXISTS (
SELECT 1
FROM marketing.leads l
WHERE l.campaign_id = c.id
)
ORDER BY campaign_id;This lesson's practice is part of Pro.
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