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PostgreSQL GROUP BY

The GROUP BY clause groups rows sharing similar values into summary rows, often used with aggregate functions like COUNT(), SUM(), AVG(), MIN(), MAX().

Key Topics

1. Basic Syntax

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

Rows are grouped by department, and the count of employees is returned for each group.

2. Grouping by Multiple Columns

SELECT department, job_title, AVG(salary)
FROM employees
GROUP BY department, job_title;

This groups rows by department and job_title, calculating the average salary for each unique combination.

Best Practices

  • Ensure columns in the SELECT either appear in GROUP BY or are used in aggregate functions.
  • Use GROUP BY for meaningful summaries and analytics on large datasets.

Key Takeaways

  • GROUP BY clusters rows with identical values in specified columns.
  • Aggregate functions operate on each group separately.
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