DateTime group by date and hour

SQL Server :

SELECT [activity_dt], count(*)
FROM table1
GROUP BY DATEPART(day, [activity_dt]), DATEPART(hour, [activity_dt]);

Oracle :

SELECT [activity_dt], count(*)
FROM table1
GROUP BY TO_CHAR(activity_dt, 'DD'), TO_CHAR(activity_dt, 'hh');

MySQL :

SELECT [activity_dt], count(*)
FROM table1
GROUP BY hour( activity_dt ) , day( activity_dt )