How to create a unique dropdown list by filtering the data of two tables as needed?

[kasun]

I have 3 tables.
1. Create post

2. Collect_group_data

3. Post submit 

"Post submit" Here in the form view the following two steps have to be done.

1. select group (Here the group list is given through ref.)

2. Select post
This is where the problem has arisen. I only want to refer to one group, one post, at a time. Simply, the posts submitted to the group should not be duplicated. In the dropdown list for selecting posts, only posts that have not been submitted to the group selected from number 1 should be displayed. How to create that list?

I followed this post and created this expression. But it was not correct.

SORT( UNIQUE( SELECT(Create Post[Post Number],AND([_THISROW].[Post Name]=[Post Name],[_THISROW].[Group Name]=[Group Name]),FALSE)),)

In addition, I also tried this expression. But it didn't go well

Create Post[Post Number]-FILTER("Post Submit",AND(([Group Name]=[_THISROW].[Group Name]),([Post Name]=[_THISROW].[Post Name])))

[SkrOYC · Accepted solution]

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@kasun wrote:

In the dropdown list for selecting posts, only posts that have not been submitted to the group selected from number 1 should be displayed. How to create that list?

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I think list subtraction is the answer

[kasun]

Your tip is precisely correct.
This expression is correct.

Create Post[ID]-SELECT(Post Submit[Post Name],([Group Name]=[_THISROW].[Group Name]))

But I have a small problem. By doing this I get to select ID. Then the post cannot be identified. What to do with it? It is easy if the picture uploaded to the post is shown with a thumbnail in the dropdown list. Do you have any idea about this?

You can see it in the picture related to the table added as "Post submit" under number 3.

[kasun]

I was able to solve the problem.
See the picture below

[SkrOYC]

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@kasun wrote:

By doing this I get to select ID. Then the post cannot be identified

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