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## What changes were proposed in this pull request? un-aliased subquery is supported by Spark SQL for a long time. Its semantic was not well defined and had 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. This PR fixes the un-aliased subquery by assigning a default alias name. After this PR, 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. ## How was this patch tested? new regression test Author: Wenchen Fan <wenchen@databricks.com> Closes #18559 from cloud-fan/sub-query. |
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