SQL Subquery Basics
SQL Subquery Basics helps analysts nest one SELECT inside another query and use its result as an input.
What & Why
Every subquery follows three mechanical rules: it's wrapped in parentheses, it runs to completion before the outer query uses its result, and it returns one of three shapes — a single value, a single column of many rows, or a whole table — which determines where it's allowed to appear.
See How It Works
BUSINESS QUESTION
Marketing wants campaigns whose spend is above the overall average.
| 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 |
EXAMPLE QUERY
SELECT
id AS campaign_id,
name,
spend
FROM marketing.campaigns
WHERE spend > (SELECT AVG(spend) FROM marketing.campaigns)
ORDER BY spend DESC, campaign_id;RESULT — campaigns above average
| campaign_id | name | spend |
|---|---|---|
| 4 | Enterprise Search | 60000.00 |
| 1 | Spring Launch | 55000.00 |
The average of the four representative campaign spends is 52,500.
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