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select A.编码,A.名称,B.数量,B.尺寸,C.质量 from A
left jion B on A.id_1=B.id_1
left jion C on B.id_2=c.id_2
select * from a
left b on a.id=b.id
left c on b.id=c.id
inner jion B on A.id_1=B.id_1
inner jion C on B.id_2=c.id_2