Definition, Use of Group by and Having Clause

Estimated study time: 12 minutes. Group your data, then filter the groups themselves.

GROUP BY and HAVING work together to summarize data by category and then filter those summaries — something a plain WHERE clause can't do on its own.

What GROUP BY Does

Groups rows that share the same value in one or more columns, so aggregate functions can be applied per group instead of across the whole table.

SELECT department, COUNT(*) AS total_employees
FROM employees
GROUP BY department;

What HAVING Does

HAVING filters groups after aggregation, unlike WHERE, which filters individual rows before grouping happens.

SELECT department, COUNT(*) AS total_employees
FROM employees
GROUP BY department
HAVING COUNT(*) > 10;

Using WHERE and HAVING Together

SELECT department, AVG(salary) AS avg_salary
FROM employees
WHERE hire_date > '2020-01-01'
GROUP BY department
HAVING AVG(salary) > 60000;

Here, WHERE filters out employees hired before 2020 first, then GROUP BY groups the remaining rows, and finally HAVING keeps only departments whose average salary exceeds 60,000.

Common Mistakes

• Trying to filter on an aggregate using WHERE instead of HAVING
• Selecting a column that isn't in GROUP BY or wrapped in an aggregate function
• Forgetting that HAVING runs after grouping, so it can reference aggregate results directly