What's the difference between DISTINCT and GROUP BY?
Quick Answer
DISTINCT removes duplicate rows from the final result set based on all selected columns. GROUP BY buckets rows into groups based on specified columns, primarily so aggregate functions (COUNT, SUM, AVG, etc.) can be computed per group. Deduplication of the grouping columns is a side effect, not its primary purpose. If you're not calling an aggregate function, SELECT DISTINCT col FROM t and SELECT col FROM t GROUP BY col return the same rows, and the optimizer often executes them identically.
Detailed Answer
-- DISTINCT: unique combinations of the selected columns
SELECT DISTINCT department FROM employees;
-- GROUP BY: same result here, but built for aggregation
SELECT department FROM employees GROUP BY department;
-- GROUP BY's real purpose: per-group aggregates
SELECT department, COUNT(*), AVG(salary)
FROM employees
GROUP BY department;
Where they diverge
DISTINCT applies to every selected column together. It can't aggregate, and it can't return a value for one row that summarizes a group of others:
-- Returns one row per DISTINCT (department, job_title) combination
SELECT DISTINCT department, job_title FROM employees;
GROUP BY lets you select the grouping column(s) plus arbitrary aggregate expressions over each group — something DISTINCT simply can't do:
SELECT department, job_title, COUNT(*) AS headcount, MAX(salary) AS top_salary
FROM employees
GROUP BY department, job_title;
Selecting a non-aggregated, non-grouped column alongside GROUP BY is a functional-dependency violation. Most engines reject it (ONLY_FULL_GROUP_BY in MySQL); older/lenient MySQL modes silently return an arbitrary value from the group instead — a frequent source of subtly wrong reports.
Performance
When there's no aggregate function involved, SELECT DISTINCT col FROM t and SELECT col FROM t GROUP BY col typically produce identical execution plans. Both need some form of sort or hash-based deduplication, and query optimizers usually recognize the equivalence. Don't assume one is inherently faster without checking EXPLAIN on your specific engine and data.