What's the difference between WHERE and HAVING?

Both clauses filter rows, but at different stages of query processing (see the logical execution order: FROM → WHERE → GROUP BY → HAVING → SELECT).

WHERE: filters rows before grouping

SELECT department, AVG(salary) AS avg_salary
FROM employees
WHERE hire_date >= '2020-01-01'   -- filters individual employee rows first
GROUP BY department;

HAVING: filters groups after aggregation

SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
HAVING AVG(salary) > 75000;       -- filters the resulting department groups

Combining both

SELECT department, COUNT(*) AS headcount
FROM employees
WHERE status = 'active'           -- row-level filter first
GROUP BY department
HAVING COUNT(*) >= 10;            -- group-level filter second

The rule of thumb

• Use WHERE for conditions on raw column values that exist before grouping.
• Use HAVING for conditions on the result of an aggregate function.
• Trying to put an aggregate condition in WHERE (WHERE COUNT(*) > 5) fails, because COUNT(*) doesn't exist until GROUP BY has executed.
• Filtering earlier with WHERE is almost always more efficient than filtering later with HAVING, because WHERE reduces the row set before the (often expensive) grouping and aggregation work happens. Never use HAVING for a condition that could be expressed in WHERE.