>From Hive manual, there is only "left semi join", no "semi join", nor "inner >semi join". >From the Database world, it is just a traditional name for this kind of join: >"LEFT semi join", as a reminder to the reader that the resultset comes out >from the LEFT table ONLY. Yong
> From: lukas.e...@datageekery.com > To: user@hive.apache.org > Subject: LEFT SEMI JOIN > Date: Tue, 13 May 2014 09:30:52 +0000 > > Hello, > > We were approached to add support for Hive SQL in jOOQ [1], which might be > useful for a greater community in general. I've gone through the Hive SQL > syntax and I've encountered this interesting clause: The LEFT SEMI JOIN > clause [2] > > Example: > SELECT a.key, a.val > FROM a LEFT SEMI JOIN b on (a.key = b.key) > > My question is: Why "LEFT"? I find this misleading. Compare this with a LEFT > OUTER JOIN, which will always return all tuples from relation A at least > once, regardless if there are any tuples in relation B matched by the JOIN > predicate in "ON". So in other words, a LEFT SEMI JOIN is completely useless > as it *should* always return ALL tuples from relation A, compared to a more > useful "SEMI JOIN" or "INNER SEMI JOIN". > > What do you think? > Lukas > > [1]: http://www.jooq.org > [2]: https://cwiki.apache.org/confluence/display/Hive/LanguageManual+Joins > > -- > Lukas Eder - Head of R&D | lukas.e...@datageekery.com | +41 44 586 82 56 > Data Geekery GmbH | Binzstrasse 23 | CH-8045 Zürich | Switzerland > > http://www.datageekery.com | Get back in control of your SQLT > > >
