A similar technique is used when applying the unique filter to the outer query, with dot notation used to indicate the field by which results are to be filtered.
在对外部查询应用惟一筛选器时可使用类似的技术，点符号将用于说明筛选结果所依据的字段。
Outer joins may have a negative impact on query performance.
外部连接可能会对查询性能产生负面影响。
This query performs an outer join to compare the sales information for 10 particular stores for January and November.
这个查询会执行一个外连接操作，以便比较从 1 月份至 11 月份的 10 个特殊商店的销售信息。
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