Databasesmedium

GROUP BY vs HAVING — what’s the difference?

Answer

GROUP BY groups rows for aggregation. HAVING filters groups after aggregation (while WHERE filters rows before grouping). Example: “only groups with count > 10”.

SELECT country, COUNT(*) AS cnt
FROM users
GROUP BY country
HAVING COUNT(*) > 10;

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): sql, group-by, having, aggregation
Data model and access patterns: dominant queries (read/write ratio, sorting, pagination).
Indexes: when they help vs hurt (write amplification, memory).
Consistency & transactions: what’s guaranteed and what can bite you.
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.

Examples

Here’s an additional example (building on the short answer):

SELECT country, COUNT(*) AS cnt
FROM users
GROUP BY country
HAVING COUNT(*) > 10;

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

When would you choose an alternative and why?
What production issues show up and how do you diagnose them?
How would you test edge cases?