Using DISTINCT along with GROUP BY in SQL Server

Question

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 Actors   Does 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

User · Answer

Perhaps 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 TableA

You 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.

User · Answer

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 b   Without DISTINCT   actors ------ a b NULL a b a b   But 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 actors   Answer   count ----- 2   Use 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  id   With DISTINCT   actors  count ------------- a       2 b       1   Without DISTINCT   actors  count ------------- a       2 b       1 b       1   For 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

[sql-server] Using DISTINCT along with GROUP BY in SQL Server

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