MySQL SUM outputs wrong value in a query consisting multiple joins - mysql

Question

I'm getting information on these tables using the following query, however defenderhit and defenderdamage SUM values are getting multiplied with the row count of first join's row number.
Table 'battles':
battle_id city_id attacker defender battle_time
1 07 6 0 1342918014
Table 'battlehits':
battle_id family_id user_id hits damage
1 0 0 1000 50000
1 6 15 108 3816
1 6 2 81 2046
1 6 1 852 1344
MySQL Query:
SELECT b.battle_id, b.city_id, b.attacker, b.defender, b.battle_time,
SUM(COALESCE(bh1.damage,0)) AS attackerdamage, SUM(COALESCE(bh2.damage,0)) AS defenderdamage,
SUM(COALESCE(bh1.hits,0)) AS attackerhit, SUM(COALESCE(bh2.hits,0)) AS defenderhit
FROM battles AS b
LEFT JOIN battlehits AS bh1 ON b.attacker = bh1.family_id
LEFT JOIN battlehits AS bh2 ON b.defender = bh2.family_id
WHERE b.battle_id=1
GROUP BY b.battle_id LIMIT 1
Result of this query is as following:
battle_id city_id attacker defender battle_time attackerdamage defenderdamage attackerhit defenderhit
1 07 6 0 1342918014 7206 150000 1041 3000
As you can see in the table data, defenderhit and defenderdamage SUM values are supposed to be 1000 and 50000, but they're multiplied by 3.
What am I doing in here? What's the problem?
Thanks in advance.

Answer 1

You are getting three rows, before the group by/sum. You have one row for each of the three attacker rows from battlehits. Each of these is paired with the same defender row from battlehits, causing the defender data to be tripled. To see this, remove the group by and limit clauses and take out the sum()s. You are effectively creating the cross product of all defenders X all attackers, and then summing.
This shows the three rows with duplicated defender data. This is a consequence of doing a join on a one to many to many relationship, instead of a one to one to one.
SELECT b.battle_id, b.city_id, b.attacker, b.defender, b.battle_time,
COALESCE(bh1.damage,0) AS attackerdamage, COALESCE(bh2.damage,0) AS defenderdamage,
COALESCE(bh1.hits,0) AS attackerhit, COALESCE(bh2.hits,0) AS defenderhit
FROM battles AS b
LEFT JOIN battlehits AS bh1 ON b.attacker = bh1.family_id
LEFT JOIN battlehits AS bh2 ON b.defender = bh2.family_id
WHERE b.battle_id=1;
Output:
battle_id city_id attacker defender battle_time attackerdamage defenderdamage attackerhit defenderhit
1 7 6 0 1342918014 3816 50000 108 1000
1 7 6 0 1342918014 2046 50000 81 1000
1 7 6 0 1342918014 1344 50000 852 1000
You need to split this into separate queries. One for the attacker sums and another for the defender sums.

Answer 2

You can emulate a SUM of distinct rows by using
(SUM(t.field_to_sum) / COUNT(t.primary_key) * COUNT(DISTINCT t.primary_key))