select case DAYOFWEEK('2020-12-13')-1
when 0 then '星期日'
when 6 then '星期六'
when 5 then '星期五'
when 4 then '星期四'
when 3 then '星期三'
when 2 then '星期二'
when 1 then '星期一'
else null end as weekday
from dual;