How to use the mysql function in the ar model in the ci framework?

group by left(ctime,10) written in ci is
$this->db->group_by('left(ctime,10)');

But the last executed sql statement is GROUP BY left(o.ctime, 10) Because the position of the quotation marks will cause an error

Is there a solution?

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group by left(ctime,10) written in ci is
$this->db->group_by('left(ctime,10)');

But the last executed sql statement is GROUP BY left(o.ctime, 10) Because the position of the quotation marks will cause an error

Is there a solution?