Combine rows/columns from two tables - JOIN clause - mysql

Question

In the link see my SQLFIDDLE and see b
CREATE TABLE Projects
(`p_id` int, `project_title` varchar(9), `l_id` int);
INSERT INTO Projects
(`p_id`, `project_title`, `l_id`)
VALUES
(1, 'A', 6),
(2, 'B', 6),
(3, 'C', 7),
(4, 'D', 8),
(5, 'E', 9),
(6, 'F', 10);
CREATE TABLE Locations
(`l_id` int, `title` varchar(9), `parent_id` int );
INSERT INTO Locations
(`l_id`, `title`, `parent_id`)
VALUES
(1, 'Country', 0),
(2, 'District1', 1),
(3, 'District2', 1),
(4, 'District3', 1),
(5, 'District4', 1),
(6, 'Loc 5', 2),
(7, 'Loc 6', 3),
(8, 'Loc 7', 3),
(9, 'Loc 8', 4),
(10, 'Loc 9', 4),
(11, 'Loc 10', 4),
(12, 'Loc 11', 5);
I would like to achieve this:
+------+-----------+-------------+
| L_ID | Title | count P_ID |
+------+-----------+-------------+
| 2 | District1 | 2 |
| 3 | District2 | 2 |
| 4 | District3 | 2 |
| 5 | District4 | 0 |
+----+------------+------+-------+
I have tried with INNER JOIN, LEFT OUTER JOIN. All i can achieve is like below and doesnt help me:
+------+-----------+----------------------+
| L_ID | Title | parent_id | counted |
+------+-----------+------------+---------+
| 6 | Loc 5 | 2 | 2 |
| 7 | Loc 6 | 3 | 2 |
| 9 | Loc 8 | 4 | 2 |
+---- -+-----------+------------+---------+
Locations table is a nested one, if this matters. I need to count projects that are in each District and also to get District name.
I tried:
SELECT l.*, COUNT(p.l_id) AS thecounted
FROM locations l
INNER JOIN projects p ON p.l_id = l.l_id
GROUP BY l.parent_id
and
SELECT l.*, COUNT(p.l_id) AS thecounted
FROM locations l
LEFT OUTER JOIN projects p on l.l_id = p.l_id
GROUP BY l.parent_id

Answer 1

Consider two joins:
select d.l_id, d.title, count(p.l_id) count_p_id
from locations d
left join locations l on l.parent_id = d.l_id
left join projects p on p.l_id = l.l_id
where d.parent_id = 0
group by d.l_id, d.title
The query starts from the list of districts (d), whose parent is 0. Then, it goes down one level to the locations (l), and looks up the corresponding projects (p). The final step is aggregation.

Answer 2

The solution of GMB returns this 1 row
l_id title count_p_id
1 Country 0
using this script version
select d.l_id, d.title, count(p.l_id) count_p_id
from locations d
left join locations l on l.parent_id = d.l_id
left join projects p on p.l_id = l.l_id
where d.parent_id = 0
group by d.l_id, d.title
We get the desired result with the slightly corrected condition
where d.parent_id = 1
Result:
l_id title count_p_id
2 District1 2
3 District2 2
4 District3 2
5 District4 0
Sorry for posting the answer, instead of a simple comment, which would be sufficient, but don't have enough reputation credits yet.