selecting multiple counts when tables not directly co-relate - sql

Question

users table:
user_id (distinct for each user)
source_id (users may have the same source)
rule tables:
white_rules
black_rules
general_rules
these tables all look the same, and have:
victim_id (co-relates to user_id from users table).
rule_id (co-relates to a different table which is not important here)
What i need is to extract the amount of total rules per-type (white,black,general) per-source_id.
example:
source_id: 5 ---> total 70 white rules, total 32 black rules, total 21 general rules
source_id: 7 ---> total 2 white rules, total 0 black rules, total 4 general rules
and so on... for all distinct sources that are listed on users table.
what i tried is:
SELECT source_id,
count(w.victim_id) as total_white,
count(b.victim_id) as total_black,
count(g.victim_id) as total_general
from users
LEFT JOIN white_rules as w ON (user_id=w.victim_id)
LEFT JOIN black_rules as b ON (user_id=b.victim_id)
LEFT JOIN general_rules as g ON (user_id=g.victim_id)
where deleted='f' and source is not null
group by source;
but the result table I get has wrong (higher) numbers than what I expect to get,
so I must be doing something wrong :)
would appreciate any hinge in the right direction.

Answer 1

You need to do your counts in subqueries, or count distinct, as your multiple 1 to many relationships are causing cross joining. I don't know your data but imagine this scenario:
Users:
User_ID | Source_ID
--------+--------------
1 | 1
White_Rules
Victim_ID | Rule_ID
----------+-------------
1 | 1
1 | 2
Black_Rules
Victim_ID | Rule_ID
----------+-------------
1 | 3
1 | 4
If you run
SELECT Users.User_ID,
Users.Source_ID,
White_Rules.Rule_ID AS WhiteRuleID,
Black_Rules.Rule_ID AS BlackRuleID
FROM Users
LEFT JOIN White_Rules
ON White_Rules.Victim_ID = Users.User_ID
LEFT JOIN Black_Rules
ON Black_Rules.Victim_ID = Users.User_ID
You will get all combinations of White_Rules.Rule_ID and Black_Rules.Rule_ID:
User_ID | Source_ID | WhiteRuleID | BlackRuleID
--------+-----------+-------------+-------------
1 | 1 | 1 | 3
1 | 1 | 2 | 4
1 | 1 | 1 | 3
1 | 1 | 2 | 4
So counting the results will return 4 white rules and 4 black rules, even though there are only 2 of each.
You should get the required results if you change your query to this:
SELECT Users.Source_ID,
SUM(COALESCE(w.TotalWhite, 0)) AS TotalWhite,
SUM(COALESCE(b.TotalBlack, 0)) AS TotalBlack,
SUM(COALESCE(g.TotalGeneral, 0)) AS TotalGeneral
FROM Users
LEFT JOIN
( SELECT Victim_ID, COUNT(*) AS TotalWhite
FROM White_Rules
GROUP BY Victim_ID
) w
ON w.Victim_ID = Users.User_ID
LEFT JOIN
( SELECT Victim_ID, COUNT(*) AS TotalBlack
FROM Black_Rules
GROUP BY Victim_ID
) b
ON b.Victim_ID = Users.User_ID
LEFT JOIN
( SELECT Victim_ID, COUNT(*) AS TotalGeneral
FROM General_Rules
GROUP BY Victim_ID
) g
ON g.Victim_ID = Users.User_ID
WHERE Deleted = 'f'
AND Source IS NOT NULL
GROUP BY Users.Source_ID
Example on SQL Fiddle
An alternative would be:
SELECT Users.Source_ID,
COUNT(Rules.TotalWhite) AS TotalWhite,
COUNT(Rules.TotalBlack) AS TotalBlack,
COUNT(Rules.TotalGeneral) AS TotalGeneral
FROM Users
LEFT JOIN
( SELECT Victim_ID, 1 AS TotalWhite, NULL AS TotalBlack, NULL AS TotalGeneral
FROM White_Rules
UNION ALL
SELECT Victim_ID, NULL AS TotalWhite, 1 AS TotalBlack, NULL AS TotalGeneral
FROM Black_Rules
UNION ALL
SELECT Victim_ID, NULL AS TotalWhite, NULL AS TotalBlack, 1 AS TotalGeneral
FROM General_Rules
) Rules
ON Rules.Victim_ID = Users.User_ID
WHERE Deleted = 'f'
AND Source IS NOT NULL
GROUP BY Users.Source_ID
Example on SQL Fiddle