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。
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.
当选择创建外部联接的选项时，联接线中的菱形图会改变，以指示联接是左外部联接、右外部联接还是完全外部联接。
When you select an option to create an outer join, the diamond graphic in the join line changes to indicate that the join is a left outer, right outer, or full outer join.
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