Sum of points and rank according to points

I am creating a game where I am calculating the user points on the basis of user activities. Everything is working fine - I got the sum of points and on the basis of points I calculated the rank. I have two columns for points calculation: pointsup (contains up points) and pointsdown (contains down points)

select sum(pointsup - pointsdown) as points 
from table1 t1
join table2 t2 
    on t2.Id = t1.id
where t1.ActivityTime BETWEEN '2015-01-01' AND '2016-01-01' 
group by t1.id 
order by points desc

Everything is fine, but the problem comes when I have to rank the users with same points. In this case, I have to consider the registration date: who registers first will come up first on the ranking. The registration date is in another table table3. I have tried to join this third table but the sum is behaving oddly, showing wrong sum calculation.

Can anyone help please?

Try this query

select a.*,b.points from table3 a
join(
select Sum(PointsUp-PointsDown) as points,t1.id
from table1 t1
  join table2 t2 on t2.Id = t1.id
where t1.ActivityTime BETWEEN '2015-01-01' AND '2016-01-01'
Group By t1.Id)b on a.id=b.id
order by b.points desc

StackExchange Database Administrators Q#105766, answer score: 4