Custom validation rule based on query results - sql

Question

I have a table that stores personnel assigned to a project and if they are removed from the project.
Each project has several people filling roles.
Several of the roles can be filled by multiple simultaneously but a few can only be filled by one person at a time.
If a person gets removed from the project, their record gets tagged with a date of removal in a specified date field.
[projectNumber][employee][role][removedDate]
[employee] and [roles] are lookup fields
I have a query that identifies the roles that should have one individual filling that role at a time.
SELECT first(projectNumber) AS pn, employee, role
FROM [Table]
WHERE projectNumber = "NUMBER" and removeddate is null and (role = 9 or role = 10 or role =2 or role =3 or role =1)
GROUP BY pn, role;
This returns the roles that can only have one person in that role, for a specific project number. If that role is not filled then nothing is returned for that record in this query.
I am trying to get the form to reference the query with a dlookup but I am not sure what I am doing.
How can I create a validation rule that checks to see if the roles are filled, per project, and returns an error if yes?
Example Table:
[projectNumber][employee][role][removedDate]
1111111 A 1
1111111 B 2
1111111 C 3
1111111 D 4
1111111 E 5
1111111 F 9 2/20/19
1111111 G 4
1111111 H 5
1111111 I 9
1111111 J 10
Here would the result of the query identifying all the roles that can only have one individual assigned to that role that have not been removed.
[projectNumber][employee][role][removedDate]
1111111 A 1
1111111 B 2
1111111 C 3
1111111 I 9
From these results I want the validation checker to reference these results if someone were to add: Employee M to project 1111111 to role 1, 2, 3, 9. It should return an error since those roles are assigned.
The validation checker would NOT return an error if someone tried to add: Employee N to project 1111111 for role 10.