Customers
--CustomerID
--FName
--SalespersonID

Salesperson
--SalespersonID
--FName

CustomerID -- FName -- SalespersonID
1 -- A -- 1
2 -- B -- 2
3 -- C -- 3
4 -- D -- 4
5 -- E -- 5
6 -- F -- 1
7 -- G -- 2
8 -- H -- 3
9 -- I -- 4
10 -- J -- 5
11 -- K -- 1
12 -- L -- 2
13 -- M -- 3
14 -- N -- 4
15 -- 0 -- 5

WITH    с AS
        (
        SELECT  *, ROW_NUMBER() OVER ORDER BY (customerID) AS rn
        FROM    customers
        ),
        s AS
        SELECT  *,
                ROW_NUMBER() OVER ORDER BY (SalespersonID) AS rn
        FROM    salesPersons
        )
SELECT  c.*, s.*
FROM    с
JOIN    s
ON      s.rn =
        (с.rn - 1) %
        (
        SELECT  COUNT(*)
        FROM    salesPersons
        ) + 1