select a.员工姓名,b.s,c.考核等级 from 员工名单表 as a,员工考核表 as c left join (select 员工ID,sum(差旅费) as s from 员工差旅费记录 group by 员工ID) as b on a.员工ID=b.员工ID where a.员工ID=c.员工ID
看下层级结构，from tbl1, tbl2 left join tbl3 必然有错。
可以改成：
select a.员工姓名,b.s,c.考核等级
from 员工名单表 as a
left join 员工考核表 as c
on a.员工ID=c.员工ID
left join (select 员工ID,sum(差旅费) as s
from 员工差旅费记录
group by 员工ID) as b
on a.员工ID=b.员工ID
感觉有问题，a 表 和 c 表 需要先关联；建议也用left join
试下楼上说的，先a表 left join c表，再leftjoin b表
不知道你这个sql的目的是什么，但是，这样用 from t1, t2 left join t3 是错误的，下面我的一种正确用法，看一下是不是你需要的
select
t.员工姓名,
b.s,
t.考核等级
from
(select a.员工ID, a.员工姓名, c.考核等级 from 员工名单表 as a, 员工考核表 as c where a.员工ID=c.员工ID) as t
left join
(select 员工ID,sum(差旅费) as s from 员工差旅费记录 group by 员工ID) as b
on t.员工ID=b.员工ID