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https://issues.apache.org/jira/browse/CALCITE-4906?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17450077#comment-17450077
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Julian Hyde commented on CALCITE-4906:
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I agree; the query should return NULL.
Calcite has two ways of expanding sub-queries - in SqlToRel and using
RexSubQuery. Do they both yield the incorrect result?
> Wrong result for scalar subquery (single value aggregation) from empty input
> ----------------------------------------------------------------------------
>
> Key: CALCITE-4906
> URL: https://issues.apache.org/jira/browse/CALCITE-4906
> Project: Calcite
> Issue Type: Bug
> Reporter: Aleksey Plekhanov
> Priority: Major
>
> Scalar subqueries from the empty input return non-nullable type and in some
> cases it leads to wrong results. For example:
> {noformat}
> SELECT (SELECT 1 FROM (SELECT NULL) WHERE 1 = 0)
> {noformat}
> Returns {{0}}, but expected {{NULL}} according to the SQL standard:
> {noformat}
> Let SS be a <scalar subquery>.
> Case:
> a) If the cardinality of SS is greater than 1 (one), then an exception
> condition is raised: cardinality violation.
> b) If the cardinality of SS is 0 (zero), then the value of the <scalar
> subquery> is the null value.
> c) Otherwise, let C be the column of <query expression> simply contained in
> SS. The value of SS is the value of C in the unique row of the result of the
> <scalar subquery>.
> {noformat}
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