sql的判断语句如何写和三表联接

试下这个select x.class,x.yeartime,x.number,y.chargename,'人数系数'=case when bnum>=number/2 then breafastnumber*1.5else breafastnumber*1.2 end from(select a.yeartime,a.class,a.number,a.breafastnumber,a.breafastnumber+b.breafastnumber bnum from a,bwhere a.class=b.class and a.yeartime=b.yeartime and b.months= 月份 ) x,c ywhere x.class=y.class and x.yeartime=y.yeartime把 月份 改成你 要的那个月如果是 字符型 要加 引号我试了可以 收起