SQL Subquery in SELECT
A subquery used to compute one extra column, not to filter anything.
What & Why
A subquery can sit directly inside the SELECT list to add a computed column — as long as it returns a single value. It doesn't filter rows; it just adds information to each one.
See How It Works
BUSINESS QUESTION
Marketing wants every campaign beside overall average campaign spend.
| 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,
(SELECT ROUND(AVG(spend), 2) FROM marketing.campaigns) AS overall_avg_spend
FROM marketing.campaigns
ORDER BY spend DESC, campaign_id;RESULT — one scalar beside every campaign
| campaign_id | name | spend | overall_avg_spend |
|---|---|---|---|
| 4 | Enterprise Search | 60000.00 | 52500.00 |
| 1 | Spring Launch | 55000.00 | 52500.00 |
| 3 | Finance Retargeting | 50000.00 | 52500.00 |
| 2 | Retention Webinar | 45000.00 | 52500.00 |
The scalar average repeats without changing campaign grain.
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