📌 TL;DR

It's strictly about timing of execution:

• WHERE: Filters raw rows before they are grouped.
• HAVING: Filters aggregated groups after the math is done.

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If you need to filter by a calculated value (like SUM or COUNT), you need HAVING.

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The Execution Flow

To understand the difference, we have to look at how the database engine actually processes the query. It doesn't read top-to-bottom; it follows a specific order of operations:

1. FROM (Find the table)
2. WHERE (Filter raw rows - "Pre-filter")
3. GROUP BY (Bucket the remaining rows)
4. HAVING (Filter the buckets - "Post-filter")
5. SELECT (Return the columns)

1. GROUP BY

Before you can filter groups, you have to make them. GROUP BY collapses multiple rows into a single row based on shared values.

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Once you group, you lose access to individual row details unless you wrap them in an aggregate function (SUM, AVG, MAX, ARRAY_AGG).

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Practical examples