Finding records not present across 3 tables - Sqlite3

Question

From the diagram provided, what I'm trying to achieve is to get all the Users of a particular workspace that are NOT associated with a task within user_task.

user_workspace table contains two FKs: user_id (from User) and workspace_id (from Workspace)

user_task table contains two FKs: user_id (From user) and task_id (from Task)

Here's the SQL I'm tinkering with to give a sense of what I'm trying to achieve:

SELECT * FROM User INNER JOIN user_workspace ON (user_workspace.user_id = User._id) 
LEFT OUTER JOIN user_task ON (user_task.task_id = Task._id) 
WHERE (user_workspace.workspace_id= ?) 
AND (user_task.task_id= ? AND user_task.user_id IS NULL)
ORDER BY User.name ASC

I think I'm close but I can't seem to get the exact result I want. Thanks for reading.

Answer 1

"Find all the Users of a particular workspace that are not associated with a specific task"

You need to move the AND user_task.task_id=? condition from the WHERE clause to the related ON.

(I think you also have a typo. ON user_task.task_id = Task._id should be ON user_task.user_id = User._id):

SELECT User.* 
FROM User 
  INNER JOIN user_workspace 
    ON  user_workspace.user_id = User._id 
  LEFT OUTER JOIN user_task 
    ON  user_task.user_id = User._id
    AND user_task.task_id = ?                <-- moved here
WHERE user_workspace.workspace_id = ?
  AND user_task.user_id IS NULL
ORDER BY User.name ASC ;

You could also use NOT EXISTS. I find the code more readable:

SELECT User.* 
FROM User 
  INNER JOIN user_workspace 
    ON  user_workspace.user_id = User._id 
WHERE user_workspace.workspace_id = ?
  AND NOT EXISTS
      ( SELECT *
        FROM user_task 
        WHERE user_task.user_id = User._id
          AND user_task.task_id = ? 
      )
ORDER BY User.name ASC ;