Is there any difference between GROUP BY and DISTINCT

sql

distinct-vs-group-by

sql-engine

performance-optimization

byNikita Barsukov·Dec 8, 2024

DISTINCT operates to eliminate duplicate values across specified columns:

SELECT DISTINCT column1 FROM table; -- Because sometimes, once is enough!

On the other side, GROUP BY is employed with aggregate functions to bundle rows that have identical values in certain columns and perform operations over these bundles:

SELECT column1, COUNT(*) FROM table GROUP BY column1; -- Bundle up, it's going to get COUNT-y!

Key rule: Employ DISTINCT for unique rows; use GROUP BY when working with aggregates like COUNT, SUM, AVG.

SQL processing and execution plans

Digging into SQL engine mechanics can help us pick between GROUP BY and DISTINCT more wisely. Here's what we need to bear in mind:

Understanding SQL engine operations

Different SQL engines might create similar execution plans for DISTINCT and a GROUP BY without aggregate functions. However, same performance is not always guaranteed. You can use EXPLAIN PLAN to peek into query execution:

EXPLAIN PLAN FOR
SELECT DISTINCT column1 FROM table; -- Like a crystal ball for your query!

SQL logical operation sequence

The sequence in which SQL processes operations matters. GROUP BY occurs before DISTINCT, which influences multi-step operations:

1. FROM
2. WHERE
3. GROUP BY
4. HAVING
5. SELECT
6. DISTINCT
7. ORDER BY

Decision basis: DISTINCT vs GROUP BY

The decision basically comes down to the need for aggregation. If data summation is your end game, favor GROUP BY. If you are on a mission to eliminate duplicates, DISTINCT comes out on top.

More complex scenarios: approaching pitfalls

Let's take a trip beyond the basics and check out complex scenarios where things could get tricky:

Aggregate functions and JOINs

SELECT a.column1, COUNT(b.column2)
FROM table1 a
INNER JOIN table2 b ON a.id = b.foreignId
GROUP BY a.column1; -- It's a COUNT-ing party!

GROUP BY really shines through when combined with aggregates and JOIN. It allows for executing intricate data analysis and establishing more complex relationships than DISTINCT.

Performance: Subqueries and the Particulars

Subqueries could introduce a performance tipping point when deciding between GROUP BY and DISTINCT. The results from a subquery used in a GROUP BY might be handled differently than DISTINCT.

Ranking and window functions

When your needs supersede uniquification or aggregation, consider ranking functions like DENSE_RANK(). This gives additional analytical power:

SELECT column1, DENSE_RANK() OVER (ORDER BY column2) AS rank
FROM table; -- Get in line everyone. Orderly chaos ensues!

Key principles for successful SQL writing

During your SQL journey, always keep these pillars in mind:

Know your data: Understand your dataset's nature and the expected results.
Purpose-driven selection: Choose between GROUP BY or DISTINCT based on whether aggregation or uniquification is required.
Performance focus: Leverage EXPLAIN PLAN to perceive potential performance impacts.
Knowledge sources: Make use of platforms like sqlmag.com and asktom.oracle.com for specialized insight.