mysql weird multiple inner join situation - php

Question

my situation:
Table "_customers"
----------------------------------------------
customer_id | name | active
----------------------------------------------
1 'customer I' 1
----------------------------------------------
2 'customer II' 0
----------------------------------------------
Table "_projects"
----------------------------------------------
project_id | project_name | customer_id
----------------------------------------------
1 'project I' 1
----------------------------------------------
2 'project II' 2
----------------------------------------------
many-to-many Table "_project_sections"
----------------------------------------------
section_id | project_id
----------------------------------------------
2 1
----------------------------------------------
3 1
----------------------------------------------
4 1
----------------------------------------------
1 2
----------------------------------------------
In my Case, i need now all Customers, which are 'active'. Also they must be from a specific Section. So, as a example, i want all active customers from the Section "1". I also figured it out to get the right data, but - and thats the weird thing - only if need sections with the id > 1. If i try to get all active customers in section 1, the Query still show me all Projects with Section_id 2,3,4...
Query
SELECT c.customer_id, c.name, ps.section_id
FROM _customers c
INNER JOIN _projects p
ON p.customer_id = c.customer_id
INNER JOIN _project_sections ps
ON ps.project_id = p.project_id
WHERE c.active = 1 AND ps.section_id = 1
GROUP BY c.name
ORDER BY LOWER(c.name)
And the wrong result:
Array
(
[customer_id] => 1
[name] => customer I
[section_id] => 1
)
Maybe someone can help me, because i dont't get it. Thank you so much!

Answer 1

You don't need a GROUP BY clause to write this query.
First, let's look at active customers and their projects.
-- All active customers
SELECT c.customer_id, c.name
FROM customers c
WHERE c.active = 1;
customer_id name
--
1 customer I
-- All active customers and their projects
SELECT c.customer_id, c.name, p.project_id, p.project_name
FROM customers c
INNER JOIN projects p ON (p.customer_id = c.customer_id)
WHERE c.active = 1;
customer_id name project_id project_name
--
1 customer I 1 project I
1 customer I 2 project II
Just one active customer, who has two projects.
Now let's look at section 1 projects.
-- All projects from section 1
SELECT ps.project_id, ps.section_id
FROM project_sections ps
WHERE ps.section_id = 1;
project_id section_id
--
2 1
-- All projects from section 1, including project name
SELECT ps.project_id, p.project_name, ps.section_id
FROM project_sections ps
INNER JOIN projects p ON (p.project_id = ps.project_id)
WHERE ps.section_id = 1;
project_id project_name section_id
--
2 project II 1
Just one section 1 project. Now put the pieces together.
-- All active customers and their projects from section 1
SELECT c.customer_id, c.name, p.project_id, p.project_name, ps.section_id
FROM customers c
INNER JOIN projects p ON (p.customer_id = c.customer_id)
INNER JOIN project_sections ps ON (p.project_id = ps.project_id)
WHERE c.active = 1 AND ps.section_id = 1;
customer_id name project_id project_name section_id
--
1 customer I 2 project II 1
That's what I'd expect.
Later . . .
I see that you've changed the data. If I make the same changes here, then my last query above returns no rows. I think that's what you said you expected. (And, with your changes to the data, that's what I'd expect, too.)

Answer 2

From your description, the query should be:
SELECT c.customer_id, c.name, ps.section_id
FROM _customers c
INNER JOIN _projects p
ON p.customer_id = c.customer_id
INNER JOIN _project_sections ps
ON ps.section_id = p.project_ID
WHERE c.active = 1 AND ps.section_id = 1
GROUP BY c.name
ORDER BY LOWER(c.name)
The constraint for the specific section goes in the WHERE clause and not in the JOIN.