如图，B和C是A表的子表，现在我想将A\B\C三表一条sql语句查询出来，，目前通过关联left join查询出来，结果为D表，我想实现E表的效果，该怎么写语句呢？
select A.ID,A.名称,BC.等级A,BC.等级B
from A
left join (select AID,等级A,null as 等级B
from B
union all
select AID,null as 类别A,等级B
from C
) BC on A.ID =BC.AID
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