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https://issues.apache.org/jira/browse/SPARK-21335?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16524527#comment-16524527
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Apache Spark commented on SPARK-21335:
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User 'cnZach' has created a pull request for this issue:
https://github.com/apache/spark/pull/21647
> support un-aliased subquery
> ---------------------------
>
> Key: SPARK-21335
> URL: https://issues.apache.org/jira/browse/SPARK-21335
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 2.3.0
> Reporter: Wenchen Fan
> Assignee: Wenchen Fan
> Priority: Major
> Labels: release-notes
> Fix For: 2.3.0
>
>
> un-aliased subquery is supported by Spark SQL for a long time. Its semantic
> was not well defined and has confusing behaviors, and it's not a standard SQL
> syntax, so we disallowed it in
> https://issues.apache.org/jira/browse/SPARK-20690 .
> However, this is a breaking change, and we do have existing queries using
> un-aliased subquery. We should add the support back and fix its semantic.
> After the fix, there is no syntax change from branch 2.2 to master, but we
> invalid a weird use case:
> {{SELECT v.i from (SELECT i FROM v)}}. Now this query will throw analysis
> exception because users should not be able to use the qualifier inside a
> subquery.
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