SQL Scalar Subquery
A subquery that returns exactly one value — nothing more, nothing less.
What & Why
A scalar subquery is a subquery guaranteed to return a single row and a single column — one value. That's what lets it sit anywhere a literal number or string could go: after =, inside SELECT, even inside a calculation.
See How It Works
BUSINESS QUESTION
Show every campaign beside the overall average spend so analysts can compare the two directly.
| 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 |
Watch one scalar value join every campaign rowStep 1 of 5
(SELECT ROUND(AVG(spend), 2) FROM marketing.campaigns) = 52,500.00
| step | rows read | scalar result |
|---|---|---|
| inner aggregate | 4 campaign rows | 52,500.00 |
The inner aggregate reduces the four campaign spends to one scalar benchmark.
EXAMPLE QUERY
SELECT
name,
spend,
(SELECT ROUND(AVG(spend), 2) FROM marketing.campaigns) AS overall_avg_spend
FROM marketing.campaigns
ORDER BY spend DESC, name;This lesson's practice is part of Pro.
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