查看是否存在笛卡尔积

select t.a, count(1) from test t group by t.a order by count(1) desc limit 20;

select a,count(*) from t group by a having count(*) >1 order by a desc;

select col1 ,sum(1) from tab where 带上条件 group by col1 having count(1)>1 order by 2 desc limit 20