SQL · Aggregations & Grouping

What is GROUP BY?

Mediumsql-12

group byaggregation

Question

What is GROUP BY?

GROUP BY combines rows into groups so aggregate functions can operate on each group.

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

Output might be:

IT       20
HR       10
Finance  15

If you select a normal column that isn't in GROUP BY and isn't aggregated:

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

SQL doesn't know which employee name to return for each department.

So most SQL dialects reject it.

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