Contiditional WHERE with two joins on same table - mysql

Question

I'm trying to create an effective query but can't get it working.
Tables:
- one table containing types of objects
- one table containing objects
Conditions:
- there can be single objects of a type
- there can be child objects of a type
- parent and child objects don't need to be of the same type
- objects can be published
- types can be published
- the results should only get pulled from a specific pool of object IDs. So i need to add AND (o.id IN (1,2,3,4)
I want a simple result list that shows how many types are published and the number of objects assigned to these types.
types
id | title | published
---------------------
1 type1 1
2 type2 1
3 type3 1
4 type4 1
5 type5 1
6 type6 0
7 type7 1
objects
id |title | type | parent | published
---------------------------------------
1 a 1 0 1
2 b 1 0 1
3 c 3 2 1
4 d 2 0 1
5 e 2 2 1
6 f 4 0 0
7 g 5 6 1
8 h 6 0 1
9 i 3 8 1
10 j 3 8 0
11 k 7 8 1
Results should be:
type1 (#2) (two singles)
type2 (#2) (one single + one child of id 2)
type3 (#3) (one child of id 2 + one published child of id 8)
type4 (#0) (one single not published)
type5 (#0) (because it's parent id 6 is not published)
type6 (#0) (because type6 is not published)
I tried this one (type publishing not included):
SELECT o.type, t.title, COUNT(t.id) AS cnt
FROM types AS t
LEFT JOIN objects AS o ON o.type = t.id
LEFT JOIN objects AS o2 ON o.id = o2.parent
WHERE o.published = 1 AND o2.published = 1
GROUP BY o.type

Answer 1

The conditions in the WHERE clause negate the "outerness" of the left joins.
Move those conditions to the ON clauses. The WHERE clause can be dropped.
Also, reference columns from t, the driving table, and count non-NULL expressions from the outer joined tables.
That will allow the query to return zero counts.
I didn't fully delve into the specification, but it looks like we want to count matching rows from o and o2.
I think something like this will get a resultset consistent with one interpretation of the specification... child o2 rows get counted under parent o type, regardless of the type on the child o2 row.
This is not tested, and I'm not fully understanding the specification...
SELECT t.id AS `type`
, t.title AS `title`
, COUNT(DISTINCT o.id)
+ COUNT(DISTINCT o2.id) AS `cnt`
-- , COUNT(DISTINCT o.id) AS `cnt_o`
-- , COUNT(DISTINCT o2.id) AS `cnt_o2`
FROM types t
LEFT
JOIN objects o
ON o.type = t.id
AND o.published = 1
AND o.parent = 0
AND t.published = 1
LEFT
JOIN objects o2
ON o2.parent = o.id
AND o2.published = 1
GROUP
BY t.id
, t.title
Not clear in the spec...
Do child rows (from o2) get omitted from the count if the type on the o2 row matches a row in types that is published=0 ?
If we are "grouping" by type on the o2 rows , then we'd need to something different,
EDIT
we could get the count from the parent and the child separately, in two separate SELECT, and then combine the two resultsets with a UNION ALL set operator, and then total up the counts.
something along these lines:
SELECT c.type
, c.title
, SUM(c.cnt) AS cnt
FROM (
SELECT t.id AS `type`
, t.title AS `title`
, COUNT(o.id) AS `cnt`
FROM types t
LEFT
JOIN objects o
ON o.type = t.id
AND o.published = 1
AND o.parent = 0
AND t.published = 1
GROUP
BY t.id
, t.title
UNION ALL
SELECT tc.id AS `type`
, tc.title AS `title`
, COUNT(oc.id) AS `cnt`
FROM types tc
JOIN objects oc
ON oc.type = t.id
AND oc.published = 1
AND t.published = 1
JOIN objects op
ON op.id = oc.parent
AND op.published = 1
JOIN types pt
ON pt.id = op.type
AND pt.published = 1
GROUP
BY tc.id
, tc.title
) c
GROUP
BY c.type
, c.title
again, untested, and without a full understanding of the spec.
the count of the parent o is straightforward. we use an outer join, with t as the driving table, so we get all types, and can get zero counts.
the count of the child oc, we can do inner joins. since the previous SELECT is getting us all the types, missing rows in the second SELECT won't cause a problem.
note that we join the child o2 rows by type, and then we join to parent (to make sure parent is published), and join to parent type (to check that type is published) ...
How do we distinguish "parent" rows, do we check parent=0 ?
Is this a hierarchy, can a "child" also be the "parent" of another row ?
FOLLOWUP
Another way to think about it (maybe this was the approach of the OP query) ... we are counting rows from o, parents and children. What's important is that the type is published type, and that o is published.
Additionally, either
o is not a child (i.e. there isn't a row in objects op that has an id value equal to `o.parent)
or
if o does have a parent row (a row in objects op with an id value equal to o.parent, the [parent op is published and the parent type is published.
We could approach it like this:
SELECT t.id AS `type`
, t.title AS `title`
, COUNT(o.id) AS `cnt`
FROM types t
LEFT
JOIN objects o
ON o.type = t.id
AND o.published = 1
AND t.published = 1
LEFT
JOIN objects op
ON op.id = o.parent
LEFT
JOIN types pt
ON pt.id = op.type
WHERE -- this not a child (there is no parent)
op.id IS NULL
OR -- parent is published and parent type is published
( op.published = 1 AND pt.published = 1 )
GROUP
BY t.id
, t.title