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CREATE TABLE users (
  id int(10) unsigned NOT NULL auto_increment,
  name varchar(255) NOT NULL,
  profession varchar(255) NOT NULL,
  employer varchar(255) NOT NULL,
  PRIMARY KEY  (id)
and I want to get all unique values of profession field, what would be faster (or recommended):
SELECT DISTINCT u.profession FROM users u
SELECT u.profession FROM users u GROUP BY u.profession
                You could test for yourself as quickly as ask the question. Irritatingly, it is almost impossible to construct a scenario in which DISTINCT outperforms GROUP BY - which is annoying because clearly this is not the purpose of GROUP BY. However, GROUP BY can produce misleading results, which I think is reason enough for avoiding it.
– Strawberry
                Aug 11, 2014 at 22:19
                There's another duplicate with a different answer. see MySql - Distinct vs Group By <<< it says GROUP BY is better
– Aung Myo Linn
                Jun 3, 2016 at 9:27
                Please see here if you want to measure the time difference between DISTINCT and GROUP BY running your query.
– Aung Myo Linn
                Jun 7, 2016 at 8:28
                Also, don't use DISTINCT and GROUP BY in the same query. It can cause huge slowness issues for some queries.
– akenney
                Feb 10 at 18:49
They are essentially equivalent to each other (in fact this is how some databases implement DISTINCT under the hood).
If one of them is faster, it's going to be DISTINCT.  This is because, although the two are the same, a query optimizer would have to catch the fact that your GROUP BY is not taking advantage of any group members, just their keys. DISTINCT makes this explicit, so you can get away with a slightly dumber optimizer.
When in doubt, test!
                DISTINCT will be faster only if you DON'T have an index (as it doesn't sort). When you do have an index and it's used, they're synonyms.
– Quassnoi
                Feb 27, 2009 at 15:11
                The definition of DISTINCT and GROUP BY differ in that DISTINCT doesn't have to sort the output, and GROUP BY by default does. However, in MySQL even a DISTINCT+ORDER BY might still be faster than a GROUP BY due to the extra hints for the optimizer as explained by SquareCog.
– rustyx
                Jan 25, 2015 at 15:03
                I tested this, and found that on an indexed column, mysql, group by was about 6x slower than distinct with a fairly complicated query. Just adding this as a datapoint. About 100k rows. So test it and see for yourselves.
– Lizardx
                Feb 21, 2016 at 22:32
If you have an index on profession, these two are synonyms.
If you don't, then use DISTINCT.
GROUP BY in MySQL sorts results. You can even do:
SELECT u.profession FROM users u GROUP BY u.profession DESC
and get your professions sorted in DESC order.
DISTINCT creates a temporary table and uses it for storing duplicates. GROUP BY does the same, but sortes the distinct results afterwards.
SELECT DISTINCT u.profession FROM users u
is faster, if you don't have an index on profession.
All of the answers above are correct, for the case of DISTINCT on a single column vs GROUP BY on a single column. 
Every db engine has its own implementation and optimizations, and if you care about the very little difference (in most cases) then you have to test against specific server AND specific version! As implementations may change...
BUT, if you select more than one column in the query, then the DISTINCT is essentially different! Because in this case it will compare ALL columns of all rows, instead of just one column.
So if you have something like:
// This will NOT return unique by [id], but unique by (id,name)
SELECT DISTINCT id, name FROM some_query_with_joins
// This will select unique by [id].
SELECT id, name FROM some_query_with_joins GROUP BY id
It is a common mistake to think that DISTINCT keyword distinguishes rows by the first column you specified, but the DISTINCT is a general keyword in this manner.
So people you have to be careful not to take the answers above as correct for all cases... You might get confused and get the wrong results while all you wanted was to optimize!
                Although this question is about MySQL it should be noted that the second query will work only in MySQL. Nearly every other DBMS will reject the second statement because it's an invalid use of the GROUP BY operator.
– a_horse_with_no_name
                Sep 15, 2013 at 10:44
                Well, "nearly" is a problematic definition :-) It would be much more helpful if you state a specific DBMS that you have tested to see that it generates an error for this statement.
– daniel.gindi
                Sep 15, 2013 at 11:53
                Postgres, Oracle, Firebird, DB2, SQL Server for starters. MySQL :sqlfiddle.com/#!2/6897c/1 Postgres: sqlfiddle.com/#!12/6897c/1 Oracle: sqlfiddle.com/#!12/6897c/1 SQL Server: sqlfiddle.com/#!6/6897c/1
– a_horse_with_no_name
                Sep 15, 2013 at 13:09
                And to confuse us :) , mysql allows using select distinct(a), b which means select distinct a, b, which means distinct on the pair.
– Marinos An
                Mar 4, 2021 at 13:01
well distinct can be slower than group by on some occasions in postgres (dont know about other dbs).
tested example:
postgres=# select count(*) from (select distinct i from g) a;
count 
10001
(1 row)
Time: 1563,109 ms
postgres=# select count(*) from (select i from g group by i) a;
count
10001
(1 row)
Time: 594,481 ms
http://www.pgsql.cz/index.php/PostgreSQL_SQL_Tricks_I
so be careful ... :)
                GROUP BY is also faster than DISTINCT in AWS Redshift, because GROUP BY uses a XN HashAggregate and DISTINCT uses a XN Unique. It is the same problem that older versions of Postgres have.
– Bernardo Loureiro
                Jul 1, 2020 at 18:03
Group by is expensive than Distinct since Group by does a sort on the result while distinct avoids it. But if you want to make group by yield the same result as distinct give order by null .. 
SELECT DISTINCT u.profession FROM users u
is equal to 
SELECT u.profession FROM users u GROUP BY u.profession order by null
describe select distinct productname from northwind.products
describe select productname from northwind.products group by productname
The second query gives additionally "Using filesort" in Extra.
                They are the same in terms of what they get, not in terms of how they get it. An ideal optimizer would execute them the same way, but MySQL optimizer is not ideal. Based on your evidence, it would seem that DISTINCT would go faster -- O(n) vs O(n*log n).
– SquareCog
                Feb 24, 2009 at 15:07
                In this case it is, because you don't need to sort (you would if you needed the groups).  MySQL sorts in order to place the same entries together, and then get groups by scanning the sorted file.  You just need distincts, so you just have to hash your keys while doing a single table scan.
– SquareCog
                Feb 25, 2009 at 15:36
opnamegroep_intern
  FROM telwerken
  WHERE opnemergroep IN (7,8,9,10,11,12,13) group by opnamegroep_intern

635 totaal 0.0944 seconds
Weergave van records 0 - 29 ( 635 totaal, query duurde 0.0484 sec)
SELECT sql_no_cache
distinct (opnamegroep_intern)
  FROM telwerken
  WHERE opnemergroep IN (7,8,9,10,11,12,13) 
635 totaal 0.2117 seconds ( almost 100% slower )
Weergave van records 0 - 29 ( 635 totaal, query duurde 0.3468 sec)
(more of a functional note)
There are cases when you have to use GROUP BY, for example if you wanted to get the number of employees per employer:
SELECT u.employer, COUNT(u.id) AS "total employees" FROM users u GROUP BY u.employer
In such a scenario DISTINCT u.employer doesn't work right. Perhaps there is a way, but I just do not know it. (If someone knows how to make such a query with DISTINCT please add a note!)
Here is a simple approach which will print the 2 different elapsed time for each query.
DECLARE @t1 DATETIME;
DECLARE @t2 DATETIME;
SET @t1 = GETDATE();
SELECT DISTINCT u.profession FROM users u; --Query with DISTINCT
SET @t2 = GETDATE();
PRINT 'Elapsed time (ms): ' + CAST(DATEDIFF(millisecond, @t1, @t2) AS varchar);
SET @t1 = GETDATE();
SELECT u.profession FROM users u GROUP BY u.profession; --Query with GROUP BY
SET @t2 = GETDATE();
PRINT 'Elapsed time (ms): ' + CAST(DATEDIFF(millisecond, @t1, @t2) AS varchar);
OR try SET STATISTICS TIME (Transact-SQL)
SET STATISTICS TIME ON;
SELECT DISTINCT u.profession FROM users u; --Query with DISTINCT
SELECT u.profession FROM users u GROUP BY u.profession; --Query with GROUP BY
SET STATISTICS TIME OFF;
It simply displays the number of milliseconds required to parse, compile, and execute each statement as below:
 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 2 ms.
This is not a rule
For each query .... try separately distinct and then group by ... compare the time to complete each query and use the faster .... 
In my project sometime I use group by and others distinct
If you don't have to do any group functions (sum, average etc in case you want to add numeric data to the table), use SELECT DISTINCT. I suspect it's faster, but i have nothing to show for it.
In any case, if you're worried about speed, create an index on the column.
If the problem allows it, try with EXISTS, since it's optimized to end as soon as a result is found (And don't buffer any response), so, if you are just trying to normalize data for a WHERE clause like this
SELECT FROM SOMETHING S WHERE S.ID IN ( SELECT DISTINCT DCR.SOMETHING_ID FROM DIFF_CARDINALITY_RELATIONSHIP DCR ) -- to keep same cardinality
A faster response would be:
SELECT FROM SOMETHING S WHERE EXISTS ( SELECT 1 FROM DIFF_CARDINALITY_RELATIONSHIP DCR WHERE DCR.SOMETHING_ID = S.ID )
This isn't always possible but when available you will see a faster response.
in mySQL i have found that GROUP BY will treat NULL as distinct, while DISTINCT does not.

Took the exact same DISTINCT query, removed the DISTINCT, and added the selected fields as the GROUP BY, and i got many more rows due to one of the fields being NULL.
So.. I tend to believe that there is more to the DISTINCT in mySQL.