MySQL Joining multiple tables each with multiple rows - mysql

Question

I have tables (bar, baz) which have 1 or more rows relating to another table (foo). When I join both bar & baz to foo I get results for each row of each table.
http://sqlfiddle.com/#!9/1c13f2/1/0
CREATE TABLE foo (`id` int, `value` varchar(5));
INSERT INTO foo (`id`, `value`) VALUES
(1, 'two'),
(2, 'two'),
(3, 'one');
CREATE TABLE bar (`id` int, `foo_id` int, `value` int);
INSERT INTO bar (`id`, `foo_id`, `value`) VALUES
(1, 1, 1),
(2, 1, 1),
(3, 2, 1),
(4, 2, 1),
(5, 3, 1);
CREATE TABLE baz (`id` int, `foo_id` int, `value` int);
INSERT INTO baz (`id`, `foo_id`, `value`) VALUES
(1, 1, 1),
(2, 1, 1),
(3, 2, 1),
(4, 2, 1),
(5, 3, 1);
The query:
SELECT foo.value, SUM(bar.value), SUM(baz.value)
FROM foo
JOIN bar ON bar.foo_id = foo.id
JOIN baz ON baz.foo_id = foo.id
GROUP BY foo.id
Result:
value SUM(bar.value) SUM(baz.value)
two 4 4
two 4 4
one 1 1
Expected result:
value SUM(bar.value) SUM(baz.value)
two 2 2
two 2 2
one 1 1

Answer 1

The result is the expected behavior, from a cross (semi-Cartesian) product, multiple rows from bar matched to multiple rows from baz.
To avoid this, we can pre-aggregate counts from bar and baz, and then do the join.
Also consider, what result is expected when there are matching rows in bar but no matching rows in baz. Do we want to return the total from bar? With the current query, we wouldn't get total from bar. (In the example data, consider what the query will return after row id=5 is deleted from baz.)
I'd write the query like this:
SELECT foo.value
, IFNULL( r.tot_bar_value ,0) AS tot_bar_value
, IFNULL( z.tot_baz_value ,0) AS tot_baz_value
FROM foo
LEFT
JOIN ( -- aggregate total from bar
SELECT bar.foo_id
, SUM(bar.value) AS tot_bar_value
FROM bar
GROUP BY bar.foo_id
) r
ON r.foo_id = foo.id
LEFT
JOIN ( -- aggregate total from bar
SELECT baz.foo_id
, SUM(baz.value) AS tot_baz_value
FROM baz
GROUP BY baz.foo_id
) z
ON z.foo_id = foo.id
Note that we are using outer joins, to handle the case when there not matching rows in either bar or baz.
For testing, we can run separately just the SELECT query inside the parens, to see what is returned.