GROUP BY

The GROUP BY clause groups rows by one or more columns and applies aggregate functions to each group.

Syntax

SELECT <column> [, ...], <aggregate_function>(<column>)
  FROM <relation_name>
 WHERE <condition>
 GROUP BY { <column> [, ...] | ALL }
 ORDER BY ...;

Parameters

<column> — one or more columns to group rows by, comma-separated. Every non-aggregated column in the SELECT list must appear here.
ALL — group by every non-aggregated column in the SELECT list automatically, equivalent to listing them explicitly.

Examples

Grouping by Columns

Group results by one or more columns:

SELECT category, COUNT(*) AS item_count
  FROM products
 GROUP BY category;

SELECT country, city, COUNT(*) AS user_count
  FROM users
 GROUP BY country, city;

GROUP BY ALL

Include all non-aggregated columns from the SELECT list automatically:

SELECT category, brand, COUNT(*) AS count
  FROM products
 GROUP BY ALL;
-- Equivalent to: GROUP BY category, brand

Single Column Grouping

SELECT status, COUNT(*) AS total
  FROM orders
 GROUP BY status
 ORDER BY total DESC;

Multiple Column Grouping

SELECT 
  EXTRACT(YEAR FROM created_at) AS year,
  category,
  COUNT(*) AS orders,
  SUM(amount) AS revenue,
  AVG(amount) AS avg_order_value
FROM orders
GROUP BY year, category
ORDER BY year DESC, revenue DESC;

Conditional Aggregation

SELECT 
  customer_id,
  COUNT(*) AS total_orders,
  SUM(CASE WHEN status = 'completed' THEN amount ELSE 0 END) AS completed_revenue
FROM orders
GROUP BY customer_id;

With HAVING Filter

SELECT category, COUNT(*) AS count
  FROM products
 GROUP BY category
 HAVING COUNT(*) > 10;

Notes

Columns in SELECT must either be in GROUP BY or used within an aggregate function.
GROUP BY ALL automatically groups by all non-aggregated columns.
Common aggregate functions used with GROUP BY: COUNT(*), SUM(column), AVG(column), MIN(column), MAX(column).
Use HAVING to filter groups after aggregation.
WHERE filters rows before grouping; HAVING filters groups after.