SQL STDDEV
The function that computes standard deviation — and the sample-vs-population choice that changes the number slightly.
What & Why
Postgres offers STDDEV_POP (treats your data as the entire population) and STDDEV_SAMP (treats it as a sample of a larger population). Plain STDDEV is an alias for STDDEV_SAMP.
See How It Works
BUSINESS QUESTION
Marketing wants spend variability by campaign channel.
| 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
channel,
ROUND(STDDEV(spend), 2) AS spend_stddev
FROM marketing.campaigns
GROUP BY channel
ORDER BY channel;RESULT — spend standard deviation
| channel | spend_stddev |
|---|---|
| NULL | |
| google_ads | 3535.53 |
| NULL |
PostgreSQL STDDEV is the sample standard deviation.
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