操作可以是一个关系运算符，或者系统或用户定义的函数，也可以是一个sql操作符，例如GROUP BY、ORDER BY等。
An operation can be a relational operator, a system - or user-defined function, or an SQL operator, such as GROUP BY, ORDER BY, and so on.
由于这一层关系，它操作起来更像是一个用户定义的关系操作符。
With the correlation it operates more like a user-defined relational operator.
部署描述符中包含的抽象模式定义了实体bean的持久性字段，以及它与其他bean之间的关系。
The abstract schema, part of the deployment descriptor, defines the persistence fields of the entity bean and its relationships with other beans.
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