Group by date only on a Datetime column

Having a table with a column like: `mydate DATETIME` ... I have a query such as: SELECT SUM(foo), mydate FROM a_table GROUP BY a_table.mydate; This will group by the full `datetime`, including hours and minutes. I wish to make the group by, only by the date `YYYY/MM/DD` not by the `YYYY/MM/DD/HH/mm`. How to do this?