SQL Nested Subqueries
A subquery inside a subquery — each layer resolves before the one wrapping it.
What & Why
Subqueries can nest more than one level deep. Postgres resolves from the innermost query outward — each layer becomes a plain value (or list, or table) for the layer around it.
See How It Works
BUSINESS QUESTION
Find campaigns above the average spend of channels that have produced 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
name,
channel,
spend
FROM marketing.campaigns
WHERE spend > (
SELECT AVG(spend)
FROM marketing.campaigns
WHERE id IN (
SELECT DISTINCT campaign_id
FROM marketing.leads
WHERE campaign_id IS NOT NULL
)
)
ORDER BY spend DESC, name;RESULT — above the 50,000 qualifying-campaign average
| name | channel | spend |
|---|---|---|
| Enterprise Search | google_ads | 60000.00 |
| Spring Launch | google_ads | 55000.00 |
The inner ID set is 1, 2, and 3, whose average spend is 50,000.
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