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https://issues.apache.org/jira/browse/SPARK-27986?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16947822#comment-16947822
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Xiao Li commented on SPARK-27986:
---------------------------------
There is a blog to describe this feature too.
[https://blog.jooq.org/2014/12/30/the-awesome-postgresql-9-4-sql2003-filter-clause-for-aggregate-functions/]
> Support Aggregate Expressions with filter
> -----------------------------------------
>
> Key: SPARK-27986
> URL: https://issues.apache.org/jira/browse/SPARK-27986
> Project: Spark
> Issue Type: Sub-task
> Components: SQL
> Affects Versions: 3.0.0
> Reporter: Yuming Wang
> Priority: Major
>
> An aggregate expression represents the application of an aggregate function
> across the rows selected by a query. An aggregate function reduces multiple
> inputs to a single output value, such as the sum or average of the inputs.
> The syntax of an aggregate expression is one of the following:
> {noformat}
> aggregate_name (expression [ , ... ] [ order_by_clause ] ) [ FILTER ( WHERE
> filter_clause ) ]
> aggregate_name (ALL expression [ , ... ] [ order_by_clause ] ) [ FILTER (
> WHERE filter_clause ) ]
> aggregate_name (DISTINCT expression [ , ... ] [ order_by_clause ] ) [ FILTER
> ( WHERE filter_clause ) ]
> aggregate_name ( * ) [ FILTER ( WHERE filter_clause ) ]
> aggregate_name ( [ expression [ , ... ] ] ) WITHIN GROUP ( order_by_clause )
> [ FILTER ( WHERE filter_clause ) ]{noformat}
> [https://www.postgresql.org/docs/current/sql-expressions.html#SYNTAX-AGGREGATES]
>
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