SELECT UserID,
       COUNT(*)
FROM yourTable
WHERE CompletedDate > DATEADD(DAY,DATEDIFF(DAY,0,GETDATE()),0)
Group by UserID

SELECT UserID, count(UserID)
FROM MyTable
WHERE DATEDIFF(day,Completed,GETDATE()) = 0
GROUP BY UserID

SELECT
        USERID,COUNT(*) AS TASK_COMPLETED
FROM
        YOUR_TABLE
WHERE
        Convert(varchar(11), COMPLETED,101) = Convert(varchar(11), getdate(),101)
GROUP BY
        USERID

select user_id,count(completed),completed from table
where completed = to_date(sysdate,'dd-mon-yy') group by completed