SQL "In" Script

Dear all, hello! I am new to Ninox and really excited about it. Great job guys.

I need 2 different things as described below:

I have a parent table who adds records in a child table…
Customers -> 'Employees in charge for the client’ . This table has a field Employees, connection from another table called Employees.

1) What code I should use in the constraint, so each time I try to add a new  'Employees in charge for the client’  in the company it doesnt appear those who I have added already in the 'Employees in charge for the client’   table?
I need this cause they are many, and Is very difficult to choose from a big list.

In SQL we would do the following: Select * from CustomerTeams where customerid = x and employeID not in (select *  from .... )

How can we put an array in our "where" statement..

2) Can I select multiple records from a table/view and add them in another table?

Thanks a lot,

Alexandros