GROUP BY Examples in SQL

What GROUP BY does

Summarize data by category, month, region or customer. SQL syntax can vary by database, but the pattern below is a useful starting point for reports and analysis.

Syntax or pattern

SELECT category, SUM(amount) FROM table_name GROUP BY category;

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5 practical examples

Sales by region

Summarize total sales for each region.

SELECT region, SUM(amount) AS total_sales
FROM sales
GROUP BY region;

GROUP BY collapses rows into one result per region.

Orders by customer

Count orders for each customer.

SELECT customer_id, COUNT(*) AS order_count
FROM orders
GROUP BY customer_id;

This is useful for customer activity analysis.

Revenue by product category

Group sales by product category.

SELECT p.category, SUM(oi.quantity * oi.unit_price) AS revenue
FROM order_items oi
JOIN products p ON oi.product_id = p.product_id
GROUP BY p.category;

Join first, then group by the reporting category.

Monthly order count

Group orders by month.

SELECT DATE_TRUNC('month', order_date) AS month, COUNT(*) AS orders
FROM orders
GROUP BY DATE_TRUNC('month', order_date);

The date expression used in SELECT usually needs to match GROUP BY.

Average order value by channel

Compare channels by average order amount.

SELECT channel, AVG(total_amount) AS avg_order_value
FROM orders
GROUP BY channel;

Averages by group are useful in ecommerce reports.

Common mistakes to avoid

Forgetting that SQL dialects vary across PostgreSQL, SQL Server, MySQL, BigQuery and SQLite.
Using SELECT * in production reports when only a few columns are needed.
Not checking join keys, duplicate rows or NULL values before trusting results.

FAQ

Will this SQL work in every database?

The idea is portable, but function names and date syntax may vary. Check your database dialect if a function is not recognized.

Should I use this in a report query?

Yes, if the pattern matches the business question and you have checked filters, joins and row counts.

Why does my result have too many rows?

The most common reasons are duplicate join keys, missing filters or grouping at the wrong level of detail.

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