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select (select count(*) from tbl where num>100) / (select count(*) from tbl where num <100);

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select (select count(*) from tbl where num>100) / (select count(*) from tbl where num <100);

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select sum(if(num>100,1,0))/sum(if(num <100,1,0)) from tbl

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select sum(if(num>100,1,0))/sum(if(num <100,1,0)) from tbl

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