如何使用最右边的表过滤多对多左连接？

Question

Suppose I have the following tables:

Machine

| id | name    |
+----+---------+
| 1  | Blender |
| 2  | Slicer  |
| 3  | Masher  |

Part

| id | name            | instock |
+----+-----------------+---------+
| 1  | Blade           | 5       |
| 2  | Rotating engine | 0       |
| 3  | Plug            | 10      |
| 4  | Wire            | 10      |

MachinePart (many-to-many)

| mid | pid |
+-----+-----+
| 1   | 1   |
| 1   | 2   |
| 1   | 3   |
| 1   | 4   |
| 2   | 1   |
| 2   | 3   |
| 2   | 4   |

I want a single query to list all machines with parts (outer joined), as long as the part is in stock .

I have tried doing the condition in the LEFT JOIN, but then the middle table generates an unwanted record:

SELECT m.id, m.name, mp.pid, p.name, p.instock
  FROM machine m
  LEFT JOIN machinepart mp ON mp.mid = m.id
  LEFT JOIN part p ON p.id = mp.pid AND instock > 0

| id | name    | pid  | name  | instock |
+----+---------+------+-------+---------+
| 1  | Blender | 1    | Blade | 5       |
| 1  | Blender | 2    | NULL  | NULL    |  -> I don't want this record
| 1  | Blender | 3    | Plug  | 10      |
| 1  | Blender | 4    | Wire  | 10      |
| 2  | Slicer  | 1    | Blade | 5       |
| 2  | Slicer  | 3    | Plug  | 10      |
| 2  | Slicer  | 4    | Wire  | 10      |
| 3  | Masher  | NULL | NULL  | NULL    |  -> But good, I want this record

How do I filter machinepart based on a field in the related table part ?

If it matters, I'm using PostgreSQL. (but I assume the solution will be generic)

Answer 1

WHERE P.INSTOCK IS NOT NULL

You can also get all of the "out of stock" parts with IS NULL instead

edit: I didn't see you DO want results for machines with no parts (because neither "part is out of stock" nor "part is in stock" make sense for a machine with no parts?)

WHERE P.INSTOCK IS NOT NULL
OR MP.PID IS NULL

edit2: <2nd suggestion won't work either>

Answer 2

I think you want an INNER JOIN and then a LEFT JOIN :

SELECT m.id, m.name, mp.pid, p.name, p.instock
FROM machine m INNER JOIN
     machinepart mp
     ON mp.mid = m.id LEFT JOIN
     part p
     ON p.id = mp.pid AND p.instock > 0;

EDIT:

This produces the results that you request:

SELECT m.id, m.name, mp.pid, p.name, p.instock
FROM machine m LEFT JOIN
     machinepart mp
     ON mp.mid = m.id LEFT JOIN
     part p
     ON p.id = mp.pid 
WHERE (p.instock > 0 OR p.instock IS NULL);

Here is a db<>fiddle.

Seems like unusual logic, though.