GROUP BY
Groups rows so aggregate functions can be computed per group.
SELECT group_col, AGG(col) FROM table_name GROUP BY group_col;This static page keeps the syntax and examples indexed for search, while the coding app handles interactive exploration and saved references.
What it does
Overview
Groups rows so aggregate functions can be computed per group.
The GROUP BY clause is a fundamental SQL operation used to arrange identical data into groups. It typically follows the WHERE clause and precedes ORDER BY in the query execution order. When executed, the database engine partitions the result set into discrete buckets based on the grouping keys. Performance is heavily influenced by indexing; grouping on indexed columns allows the engine to perform a 'Group Aggregate' or 'Stream Aggregate' (sequential read) rather than a more expensive 'Hash Aggregate' which requires building an in-memory hash table. A critical edge case involves NULL values: standard SQL treats all NULL values as a single group. Non-aggregated columns in the SELECT list must be included in the GROUP BY clause to avoid indeterminate results, a requirement strictly enforced by most modern RDBMS like PostgreSQL and SQL Server, although some configurations of MySQL allow the omission of non-aggregated columns if functional dependency is maintained.
Quick reference
Syntax
SELECT group_col, AGG(col) FROM table_name GROUP BY group_col;Inputs
Parameters
See it in practice
Examples
Basic Count Aggregation
SELECT department, COUNT(*) AS employee_count FROM employees GROUP BY department;department | employee_count Sales | 15 Tech | 42 HR | 8
Groups employees by their department and counts the number of records in each bucket.
Grouping by Multiple Columns
SELECT region, product_category, SUM(revenue) FROM sales_data GROUP BY region, product_category;region | product_category | sum North | Electronics | 55000 North | Furniture | 12000 South | Electronics | 48000
Calculates revenue for every unique combination of region and category.
Filtering Groups with HAVING
SELECT supplier_id, AVG(lead_time) FROM orders GROUP BY supplier_id HAVING AVG(lead_time) > 5;supplier_id | avg 101 | 7.2 304 | 6.5
Groups orders by supplier and filters out groups where the average lead time is 5 days or less.
Debug faster
Common Errors
IndeterminateColumnError
Cause: Attempting to select a column that is neither part of the GROUP BY clause nor wrapped in an aggregate function.
Fix: Add the missing column to the GROUP BY clause or use an aggregate like MIN() or MAX().
SELECT name, department, SUM(salary) FROM employees GROUP BY department;LogicError
Cause: Attempting to filter aggregated results inside the WHERE clause instead of the HAVING clause.
Fix: Move conditions involving aggregate functions (like SUM, COUNT) to a HAVING clause after the GROUP BY.
SELECT category FROM items WHERE COUNT(*) > 10 GROUP BY category;Runtime support
Compatibility
Date extraction functions vary by DB
Source: MDN Web Docs
Common questions
Frequently Asked Questions
Groups rows so aggregate functions can be computed per group.
ol: Column(s) that define each group. AGG(col): Aggregate calculation per group.
IndeterminateColumnError: Add the missing column to the GROUP BY clause or use an aggregate like MIN() or MAX(). LogicError: Move conditions involving aggregate functions (like SUM, COUNT) to a HAVING clause after the GROUP BY.