My DB has three tables:

CREATE TABLE Users (
    id INTEGER PRIMARY KEY,
    name TEXT NOT NULL
);

CREATE TABLE Categories (
    id INTEGER PRIMARY KEY,
    name TEXT NOT NULL,
    user_id INTEGER NOT NULL,
    FOREIGN KEY (user_id) REFERENCES Users(Id)
);

CREATE TABLE Notes (
    id INTEGER PRIMARY KEY,
    description TEXT NOT NULL,
    category_id INTEGER,
    user_id INTEGER NOT NULL,
    FOREIGN KEY (category_id) REFERENCES Categories(Id),
    FOREIGN KEY (user_id) REFERENCES Users(Id)
);

I want to implement functionality to get user an ability to share note he created with another user. I ended up with a solution to have junction table:

CREATE TABLE SharedNoteToUserId
(
    note_id int NOT NULL,
    user_id int NOT NULL,
    CONSTRAINT PK_SharedNoteToUserId PRIMARY KEY
    (
        note_id,
        user_id
    ),
    FOREIGN KEY (note_id) REFERENCES Notes(Id),
    FOREIGN KEY (user_id) REFERENCES Users(Id)
);

But I also want to have endpoint to retrieve list of all categories including categories from shared notes. It looks like it’s just union of two tables:

SELECT Categories.id, Categories.name
    FROM Categories 
    WHERE Categories.user_id = 2
UNION
SELECT Categories.id, Categories.name
    FROM SharedNoteToUserId
    JOIN Notes ON Notes.id = SharedNoteToUserId.note_id
    JOIN Categories ON Categories.id = Notes.category_id
    WHERE SharedNoteToUserId.user_id = 2;

But I’m not sure that it’s a right approach. Maybe I need to create and manage separate junction table for categories as well?