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https://issues.apache.org/jira/browse/DERBY-4374?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12761498#action_12761498
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Knut Anders Hatlen commented on DERBY-4374:
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The CROSS JOIN operator can be replaced with an INNER JOIN where the join
clause always evaluates to true, e.g. 1=1. It can also be replaced with a
sub-query. So equivalent queries will be:
INNER JOIN:
SELECT * FROM CITIES LEFT OUTER JOIN
FLIGHTS INNER JOIN COUNTRIES ON 1=1
ON CITIES.AIRPORT = FLIGHTS.ORIG_AIRPORT
WHERE COUNTRIES.COUNTRY_ISO_CODE = 'US'
Sub-query:
SELECT * FROM CITIES LEFT OUTER JOIN
(SELECT * FROM FLIGHTS, COUNTRIES) S
ON CITIES.AIRPORT = S.ORIG_AIRPORT
WHERE S.COUNTRY_ISO_CODE = 'US'
(Curiously, the queries that use CROSS JOIN or INNER JOIN need more than three
minutes to complete on the toursdb database, whereas the sub-query variant
completes in less than ten seconds. Probably an optimizer issue, but at least
the results are the same.)
> Document the CROSS JOIN operation in the reference manual
> ---------------------------------------------------------
>
> Key: DERBY-4374
> URL: https://issues.apache.org/jira/browse/DERBY-4374
> Project: Derby
> Issue Type: Improvement
> Components: Documentation
> Affects Versions: 10.6.0.0
> Reporter: Knut Anders Hatlen
> Assignee: Kim Haase
> Attachments: DERBY-4374-2.diff, DERBY-4374-2.zip, DERBY-4374.diff,
> DERBY-4374.stat, DERBY-4374.zip
>
>
> CROSS JOIN was added in DERBY-4355. We should document it alongside the other
> join operations in the reference manual.
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