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As shown in JDOQL Reference DataNucleus supports queries using the
JDOQL query language, using a Java-like syntax. When using JDOQL under RDBMS there are some specific
situations where it can be useful to benefit from special treatment. These are listed here.
When using the method
contains
on a collection (or
containsKey
,
containsValue
on a map) this will either add an EXISTS subquery (if there is a NOT or OR present in the query)
or will add an INNER JOIN across to the element table. Let's take an example
SELECT FROM org.datanucleus.samples.A
WHERE (elements.contains(b1) && b1.name == 'Jones')
VARIABLES org.datanucleus.samples.B b1
Note that we add the
contains
first that binds the variable "b1" to the element table,
and then add the condition on the variable. The order is important here. If we instead had put
the condition on the variable first we would have had to do a CROSS JOIN to the variable table
and then try to repair the situation and change it to INNER JOIN if possible. In this case the
generated SQL will be like
SELECT `A0`.`ID`
FROM `A` `A0`
INNER JOIN `B` `B0` ON `A0`.ID = `B`.ELEMENT
WHERE `B0`.NAME = 'Jones' |
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