sql选取相同的行

一个表有50万行
怎么选取相同的行？

select * from table1 a where (select count(1) from table1 where col1=a.col1)>1

太慢了，好久了还木有出来~
求招式阿~

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试试这个呢
SELECT * FROM table GROUP BY col HAVING count(col)>1