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https://issues.apache.org/jira/browse/CALCITE-4484?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17277518#comment-17277518
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Rui Wang commented on CALCITE-4484:
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+1 thanks for creating this bug [~julianhyde]
> Add UNIQUE_VALUE(x) aggregate function, that throws if x is not unique
> ----------------------------------------------------------------------
>
> Key: CALCITE-4484
> URL: https://issues.apache.org/jira/browse/CALCITE-4484
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Priority: Major
>
> Add a {{UNIQUE_VALUE\(x)}} aggregate function, that throws if {{x}} is not
> unique.
> {{UNIQUE_VALUE\(x)}} would throw if {{x}} has values [1, 2], or has values
> [1, NULL]; but would not throw if x has values [1, 1, 1] or [] or [NULL,
> NULL]. Like {{ANY_VALUE}} it behaves as if {{RESPECT NULLS}} is specified.
> There are similar functions:
> * {{ANY_VALUE\(x)}} non-deterministically picks a value. (It is present in
> BigQuery, MySQL, Snowflake, MSSQL and perhaps others.)
> * {{SINGLE_VALUE\(x)}} returns the value of x if there is just one value
> (e.g. [1] or [NULL]), NULL if there are no values, throws if there is more
> than one value (e.g. [NULL, NULL] or [1, 1, 1] or [1, 2]). {{SINGLE_VALUE}}
> is in Calcite, no other DBs that I am aware of, not documented, but available
> through SQL. Calcite uses it internally to enforce scalar sub-queries.
> BigQuery has an internal function "{{$ANY_AND_CHECK\(x)}}" that is equivalent
> to {{UNIQUE_VALUE\(x)}}.
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