Question

如何完成这个完整的外连接？

注意：我问的问题不同于original post。

TABLE1

+----+-----------+-----------+---------+
| ID | FILTER1   | FILTER2   | DATA1   |
| 1  | NULL      | filter2-A | data1-A |
| 2  | filter1-B | filter2-B | data1-B |
| 3  | filter1-C | filter2-C | data1-C |
| 3  | filter1-D | filter2-D | data1-D |
+----+-----------+-----------+---------+

TABLE2

+----+-----------+-----------+---------+
| ID | FILTER1   | FILTER2   | DATA2   |
| 1  | filter1-A | filter2-A | data2-A |
| 2  | filter1-B | filter2-B | data2-B |
| 3  | filter1-C | filter2-C | data2-C |
| 3  | filter1-D | filter2-D | data2-D |
+----+-----------+-----------+---------+

结果

+------------+-----------+---------+---------+
| FILTER1    | FILTER2   | DATA1   | DATA2   |
| NULL       | filter2-A | data1-A | NULL    |
| filter1-A  | filter2-A | NULL    | data2-A |
| filter1-B  | filter2-B | data1-B | data2-B |
| filter1-C  | filter2-C | data1-C | data2-C |
| filter1-D  | filter2-D | data1-D | data2-D |
+------------+-----------+---------+---------+

一些担忧：

这两个表实际上共享五个过滤字段，我必须对结果进行一些过滤，这意味着我需要经常引用过滤字段。而且我不希望到处都是非常冗长的COALESCE()或ISNULL()。没有求助于子查询的任何方法可以避免这种情况吗？

Answer 1

我会重复我对这个问题的回答。

如果表格中没有重复的键值（对）（如示例数据中所示），则可以使用union all和group by执行此操作：

select filter_1, filter_2, max(data_1) as data_1, max(data_2) as data_2
from ((select filter_1, filter_2, data_1, NULL as data_2
       from table1
      ) union all
      (select filter_1, filter_2, NULL, data_2
       from table2
      )
     ) t
group by filter_1, filter_2;

如何在可能的null键上进行完全外连接？

1 个答案: