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System Design Masterclass · Data Governance Compliance
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Your database has 2 million individual order rows. Your CEO does not want to see 2 million rows. They want "revenue by month," "orders per country," and "average order value by product category." These are all grouping operations: taking millions of individual records and collapsing them into meaningful summaries.
Grouping is the bridge between raw data and insights. Every chart on every dashboard is a grouping operation. Every "total sales by region" report. Every "users by signup month" metric. Every "top 10 products by revenue" list.
Airbnb's analytics team runs over 50,000 GROUP BY queries per day against their . Spotify Wrapped, the annual summary of your listening habits, is fundamentally a massive grouping pipeline: group your plays by artist, by genre, by month, by time of day, aggregate counts, rank them, and present the results as a story.
Without grouping, data is a pile of individual facts. With grouping, patterns emerge: seasonal trends, geographic hotspots, product performance, user behavior shifts.
Grouping combines rows that share a common value and applies an aggregate function to each group.
| Aggregate Function | What It Does | Example |
|---|---|---|
COUNT(*) | How many rows in each group | Orders per customer |
SUM(amount) | Total of a numeric column | Revenue per month |
AVG(price) | Average of a numeric column | Average order value per category |
MIN(created_at) | Smallest/earliest value | First order date per customer |
MAX(score) | Largest/latest value | Highest quiz score per user |
COUNT(DISTINCT col) | Unique values in each group | Unique customers per region |
Multi-level grouping: GROUP BY country, city creates groups within groups. First by country, then by city within each country. The result: revenue for Mumbai, Delhi, Bangalore (within India), then London, Manchester (within UK).
HAVING vs. WHERE:
WHERE filters individual rows before grouping.HAVING filters groups after grouping.GROUP BY country HAVING COUNT(*) > 1000WHERE COUNT(*) > 1000 is a syntax error. Aggregates can only appear in HAVING or SELECT.