update copy_emp1,(
select sal,CASE
when sal>5000 and sal <= 8000 then (sal-5000)0.03
when sal>8000 and sal<= 17000 then (sal-5000)0.1-90
when sal>17000 and sal <=30000 then (sal-5000)0.2-990
when sal>30000 and sal <=40000 then (sal-5000)0.25-3590
else 0 end tax from copy_emp1) taxtable set copy_emp1.sal=taxtable.tax where copy_emp1.sal=taxtable.sal;
！难点
