[GitHub] [spark] bart-samwel commented on pull request #28836: [SPARK-31561][SQL] Add QUALIFY Clause

[GitBox <gi...@apache.org>]

bart-samwel commented on pull request #28836:
URL: https://github.com/apache/spark/pull/28836#issuecomment-644615299


   I'm not a huge fan of this. For one thing, it is not in ANSI SQL 2016. AFAICT it's a Teradata extension that Snowflake adopted as well because they get a lot of Teradata workloads. To be honest, I find it confusing. There's already WHERE and HAVING, and QUALIFY follows HAVING but the evaluation order is different:
   
   Syntax: SELECT -> FROM -> WHERE -> GROUP BY -> **HAVING -> QUALIFY**
   Semantics: FROM -> WHERE -> GROUP BY -> **HAVING -> SELECT -> QUALIFY**
   
   And all it does is basically (SELECT * FROM (SELECT -> FROM -> WHERE -> GROUP BY -> HAVING) WHERE ...). To make this work they've invented a third synonym that basically means "filter", but the meaning is less clear than any of the other synonyms because the order in the syntax is right next to HAVING.
   


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