case when用法(casewhen怎么用嵌套查询)

selecta,casewhena>=85then'优秀'whena>=75then'良好'whena>=60then'及格'else'不及格'end,casewhenb>=85then'优秀'whenb>=75then'良好'whenb>=60then'及格'else'不及格'endbfromtable_