left join 条件,该怎么解决

left join 条件left joinon条件是个判断on ( case whenvm.invcode then bom.invcodevm.invcode else

left join 条件
left join
on
条件是个判断
on ( case when vm.invcode<>'' then bom.invcode=vm.invcode else bom.invcode=sa.cinvcode end)

这sql应该怎么写