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select a.*, b.*,c.* from a LEFT join b on a.dm=b.dm LEFT join c on a.mc =c.mc and a.dm = c.mc
select a.*, b.*,c.* from a LEFT join (select dm,min(你需要的列名),min(你需要的列名) from b group by dm) b on a.dm=b.dm LEFT join (select mc,min(你需要的列名),min(你需要的列名) from c group by mc) c on a.mc =c.mc and a.dm = c.mc