left join produces wrong results - mysql

Question

I have three tables as :
ab, a, and b
Table a and b should have multiple occurrences for the same touid.
SELECT * FROM ab
tourid tourname
------ ------------
100 hundred
110 one ten
120 one twenty
select * from a;
imageid tourid filesize
------- ------ ----------
1 100 10
2 100 20
SELECT * FROM b
uid tourid filesize
------ ------ ----------
5 100 5
sql query :
SELECT
a.tourid,
SUM(a.filesize) AS a_sum,
SUM(b.filesize) AS b_sum
FROM ab
LEFT JOIN a ON a.tourid=ab.tourid
LEFT JOIN b ON b.tourid=ab.tourid
WHERE ab.tourid=100
gives the result as:
tourid a_sum b_sum
------ ------ --------
100 30 10
But result should be as :
tourid a_sum b_sum
------ ------ --------
100 30 5
result of b_sum column is wrong

Answer 1

In the first table, you have one row :
tourid tourname
100 hundred
Joining the a table will produce 2 rows :
tourid tourname imaageid filesize
100 hundred 1 10
100 hundred 2 20
Joining the b table will keep 2 rows :
tourid tourname imaageid filesize uid filesize
100 hundred 1 10 5 5
100 hundred 2 20 5 5
the good query is :
select tourid, sum_a, sum_b
from ab
left join (select tourid, sum(filesize) as sum_a from a group by tourid) a on ab.tourid = a.tourid
left join (select tourid, sum(filesize) as sum_b from b group by tourid) b on ab.tourid = b.tourid

Answer 2

You need to add GROUP BY ab.tourid to overcome duplicates, there are repeated results therefore you are getting the sum as twice
SELECT
a.tourid,
SUM(a.filesize) AS a_sum,
SUM(b.filesize) AS b_sum
FROM ab
LEFT JOIN a ON a.tourid=ab.tourid
LEFT JOIN b ON b.tourid=ab.tourid
WHERE ab.tourid=100
GROUP BY ab.tourid

Answer 3

The counts are correct: since you are joining to both tables at once, the results before aggregation will look like this:
tourid a.filesize b.filesize
------ ---------- ----------
100 10 5
100 20 5
You get the same row of b for each joined row in a.
When you total up a.filesize, you get 30; when you total up b.filesize, you get 10, explaining the results.
You can get the results that you wish without joins or aggregations - simple subqueries would be sufficient:
SELECT
100
, (SELECT SUM(filesize) FROM a WHERE tourid=100) as sum_a
, (SELECT SUM(filesize) FROM b WHERE tourid=100) as sum_a