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https://issues.apache.org/jira/browse/CALCITE-4763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17417258#comment-17417258
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duan xiong commented on CALCITE-4763:
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[~julianhyde] [~wojustme] Thanks for your review. That's very helpful, I will
make some improvements.
> add EXISTS_AGG aggregate function
> ---------------------------------
>
> Key: CALCITE-4763
> URL: https://issues.apache.org/jira/browse/CALCITE-4763
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Assignee: duan xiong
> Priority: Major
>
> Add {{EXISTS_AGG}}, an aggregate function that returns whether count is
> greater than zero.
> Thus {{EXISTS_AGG\(\*)}} is equivalent to {{COUNT\(\*) > 0}}, and
> {{EXISTS_AGG(c)}} is equivalent to {{COUNT(c) > 0}}.
> {{EXISTS_AGG}} would mainly be of use internally. Since it produces a
> {{BOOLEAN}} value, we can use the value directly from an {{Aggregate}}
> without an intervening {{Project}}. It also captures the fact that we don't
> care how many rows were produced.
> See also {{TRUE_AGG}}, CALCITE-4334.
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