Question

I’m trying to select some data from a task table, but not any subtasks which a user may have created for themselves. So, I want to filter out any tasks that have a parent_taskid which is a task_id already assigned to that user.

E.g.

UserID | Parent_TaskID | TaskID
------ | ------------- | ------
435    | 149329        | 161280  
435    | 149330        | 210717  
435    | 149330        | 228100  
435    | 156991        | 149330  
169    | 161280        | 546540  
169    | 456842        | 458764

So from the table above TaskIDs 210717 & 228100 would be removed from my select because their parent (149330) is a taskID already assigned to that user – making them subtasks. – but 546540 would not be removed because it is a taskID assigned to another user.

So I’m thinking something like

select Task.taskID, Task.Parent_taskID, Task.userID 
from task
where Task.Parent_TaskID not in (??? select taskID from task where ???)  

Any ideas?

Answer 1

Your NOT IN will be

select t.taskID, t.Parent_taskID, t.userID 
from task t
where t.Parent_taskID not in (
    select tp.taskID
    from task tp
    where tp.userID = t.userID 
)

another good (and readable) solution is to use the NOT EXISTS

select t.taskID, t.Parent_taskID, t.userID 
from task t
where not exists (
    select 1
    from task tp
    where tp.taskID = t.Parent_taskID
      and tp.userID = t.userID 
)