PostgreSQL GROUP BY Clause
GROUP BY
The GROUP BY clause groups rows that have the same values into summary rows,
like "find the number of customers in each country".
The GROUP BY clause is often used with aggregate functions like
COUNT(),
MAX(),
MIN(),
SUM(),
AVG() to group the result-set by one or more columns.
Example
Lists the number of customers in each country:
SELECT COUNT(customer_id), country
FROM customers
GROUP BY country;
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GROUP BY With JOIN
The following SQL statement lists the number of orders made by each customer:
Example
SELECT customers.customer_name, COUNT(orders.order_id)
FROM orders
LEFT JOIN customers ON orders.customer_id = customers.customer_id
GROUP BY customer_name;
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