select a.*,case when b.column is not null then 'ture' else 'false' end
from a left join b on a.column=b.column;
其中a.column=b.column就是连接关系
select a.*,a.column,'true' from a,b where a.column=b.column
union all
select a.*,a.column,'false' from a where not exists(select b.column from b where a.column=b.column)
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