Is there any purpose for using both DISTINCT and GROUP BY in SQL?
Below is a sample code
SELECT DISTINCT Actors
FROM MovieDetails
GROUP BY ActorsDoes anyone know of any situations where both DISTINCT and GROUP BY need to be used, to get any specific desired results?
(The general usage of DISTINCT and GROUP BY separately is understood)
53 Answers
Use DISTINCT to remove duplicate GROUPING SETS from the GROUP BY clause
In a completely silly example using GROUPING SETS() in general (or the special grouping sets ROLLUP() or CUBE() in particular), you could use DISTINCT in order to remove the duplicate values produced by the grouping sets again:
SELECT DISTINCT actors
FROM (VALUES('a'), ('a'), ('b'), ('b')) t(actors)
GROUP BY CUBE(actors, actors)With DISTINCT:
actors
------
NULL
a
bWithout DISTINCT:
actors
------
a
b
NULL
a
b
a
bBut why, apart from making an academic point, would you do that?
Use DISTINCT to find unique aggregate function values
In a less far-fetched example, you might be interested in the DISTINCT aggregated values, such as, how many different duplicate numbers of actors are there?
SELECT DISTINCT COUNT(*)
FROM (VALUES('a'), ('a'), ('b'), ('b')) t(actors)
GROUP BY actorsAnswer:
count
-----
2Use DISTINCT to remove duplicates with more than one GROUP BY column
Another case, of course, is this one:
SELECT DISTINCT actors, COUNT(*)
FROM (VALUES('a', 1), ('a', 1), ('b', 1), ('b', 2)) t(actors, id)
GROUP BY actors, idWith DISTINCT:
actors count
-------------
a 2
b 1Without DISTINCT:
actors count
-------------
a 2
b 1
b 1For more details, I've written some blog posts, e.g. about GROUPING SETS and how they influence the GROUP BY operation, or about the logical order of SQL operations (as opposed to the lexical order of operations).
I would group by in a subselect and then take the distinct in the select statement:
SELECT DISTINCT *
FROM ( SELECT Actors FROM MovieDetails GROUP BY Actors ) dPerhaps not in the context that you have it, but you could use
SELECT DISTINCT col1,
PERCENTILE_CONT(col2) WITHIN GROUP (ORDER BY col2) OVER (PARTITION BY col1),
PERCENTILE_CONT(col2) WITHIN GROUP (ORDER BY col2) OVER (PARTITION BY col1, col3),
FROM TableAYou would use this to return different levels of aggregation returned in a single row. The use case would be for when a single grouping would not suffice all of the aggregates needed.
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