与和和相加。

SELECT NAME,SUM (TIME) FROM TASKS LEFT JOIN REPORT ON TASKS.ID = REPORT.TASKID                         
                GROUP BY TASKS.NAME

SELECT
  NAME, 
  SUM( ISNULL(TIME, 0) ) SumOfTime /* Time could be NULL! */
FROM
  TASKS 
  LEFT JOIN REPORT ON TASKS.ID = REPORT.TASKID                         
GROUP BY
  TASKS.NAME