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。

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。

The combined record is much easier to use-you only have to check the PICTURE field to see if it's not null (which was the result of the LEFT OUTER JOIN).

而组合的记录使用起来要容易得多—只需检查PICTURE字段，查看它是否为空(这是LEFT OUTER JOIN的结果)。

The following illustration shows a conceptual view of two sets and the elements within those sets that are included in either an inner join or a left outer join.

下面的概念图显示了两个集合中的完整项目，以及这两个集合彼此进行内部联结或左外部联结之后剩下的项目。