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group by clause query

Sudarshan Sreenivasan
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Posts: 188
IntelliJ IDE Java MySQL Database
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The page 216 of Apress - Pro EJB 3 - Java Persistence API
The GROUP BY clause defines the grouping expressions over which the results will be aggregated.
A grouping expression must either be a single-valued path expression (state field or
single-valued association field) or an identification variable. If an identification variable is
used, the entity must not have any serialized state or large object fields.

I do not quite get what "serialized state" implies in this context ??

Does it mean that an entity cannot contain properties that implement the Serializable interface ??

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