一天内的工作时长计算

同一天，不考虑法定节假日下计算非加班(8-12，14-20)工作时长

(case when finish_time <= 14 then 
   least(finish_time-create_time,finish_time-8,12-create_time,4)

case when create_time >= 12 then
    least(finish_time-create_time,finish_time -14,20-create_time,6)

case when create_time <= 8 or finish_time >= 20 then least(12-create_time,4)+least(finish_time-14,6) 
else finish_time-create_time - 2)