For all tables listed in a FROM clause, there can be, at most, one access or join request with the 'FIRST' attribute.
对于from子句中列出的所有表,最多有一个访问或计划可以带有“FIRST”属性。
Each request type should have a corresponding action element describing how to process the request. On a join request.
每个请求类型都应该有相应的action元素,用来描述如何处理该请求。
The paths then join up, and process the request.
然后将路径联合起来并处理请求。
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