In this regard, the query is similar to a left outer join in SQL.

从这种意义上说，查询与 SQL 中的左外连接相似。

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Figure 8 shows that the left outer join operations are cascaded if a rule USES more than one reference column.

图8展示了如果规则使用了多个参考列，那么左外部连接就进行级联。

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A left outer join is a join in which each element of the first collection is returned, regardless of whether it has any correlated elements in the second collection.

左外部联接是这样一个联接：在其中返回第一个集合的每个元素，而无论该元素在第二个集合中是否具有相关元素。

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