遇到一个sql有关问题

遇到一个sql问题，求助
表a：userid，username
表b：userid，incost，
表c:userid,outcost
现在要查出这样一个结果集：userid ,username,sum(incost),sum(outcost)
我的sql如下：
select a.userid,(a.username),sum(b.incost) as sumincost,sum(c.outcost) as sumoutcost 
from xxzx_user_zyl a,xxzx_incost b, xxzx_outcost c
where a.userid=b.userid and b.userid=c.userid 
group by a.userid,a.username

结果查出来的结果总是不对，求高人指导

[解决办法]
userid是否有重复的，
这个应该用full join 吧
[解决办法]

SQL code

select a.userid,       a.username,       sum(b.incost) sumincost,       sum(c.outcost) sumoutcost  from xxzx_user_zyl a,xxzx_incost b, xxzx_outcost cwhere a.userid=b.userid       and b.userid=c.userid  group by a.userid,a.username
[解决办法]
SQL code
SELECT userid,username,Sum(incost) over (PARTITION BY userid,username) as incost,Sum(outcost) over (PARTITION BY userid,username) as outcost FROM xxzx_user_zyl a,xxzx_incost b, xxzx_outcost cWHERE a.userid=b.userid(+)AND a.userid=c.userid(+)
[解决办法]
select a.userid,        a.username,        sum(b.incost) sumincost,        sum(c.outcost) sumoutcost   from xxzx_user_zyl a,xxzx_incost b, xxzx_outcost c where a.userid=b.userid(+)        and A.userid=c.userid(+)   group by a.userid,a.username
是以A表为准,进行外关联