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if exists(select 1 from a left join b on a.xxx=b.xxx whereb.aaa is null)
select * from a left join b on a.xxx=b.xxx whereb.aaa is null
else
select * from a left join b on a.xxx=b.xxx where b.aaa <='xxxx'
select * from a
right join b on a.xxx=b.xxx and b.aaa is null
where b.aaa <='xxxx'