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表A(key,v1),表B(key,v2)，其中连接的公共字段key。假设A为左表，在MySQL语言中能实现如图涂色部分查询的是（）

A. select * from A full join B using(key);

B. select * from A left join B using(key)union select * from B right join B using(key);

C. select * from A left join B on A.key=B.key where B.key is null union select * from A right join B on A.key=B.key where A.key is null;

D. select * from A left join B using(key)union all select * from B right join B using(key);

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先查出左反连接和右反连接，再对两张宽表进行纵向连接。select * from A left join B on A.key=B.key where B.key is null是左反连接，select * from A right join B on A.key=B.key where A.key is null是右反连接。