If my salesmen and customers are as follows:

```sql
--- seed salesmen table
INSERT INTO salesmen (salesman_id, name, city, commission)
VALUES
(5001, 'James Hoog', 'New York', 0.15),
(5002, 'Nail Knite', 'Paris', 0.13),
(5005, 'Pit Alex', 'London', 0.11),
(5006, 'Mc Lyon', 'Paris', 0.14),
(5007, 'Paul Adam', 'Rome', 0.13),
(5003, 'Lauson Hen', 'San Jose', 0.12);
(6 rows)
--- seed cutomers table

INSERT INTO customers (customer_id, cust_name, city, grade, salesman_id )
VALUES
(3002, 'Nick Rimando', 'New York', 100, 5001),
(3007, 'Brad Davis', 'New York', 200, 5001),
(3005, 'Graham Zusi', 'California', 200, 5002),
(3008, 'Julian Green', 'London', 300, 5002),
(3004, 'Fabian Johnson', 'Paris', 300, 5006),
(3009, 'Geoff Cameron', 'Berlin', 100, 5003),
(3003, 'Jozy Altidor', 'Moscow', 200, 5007),
(3001, 'Brad Guzan'

Then all the followin SQL statement will produce same result.

SELECT *  FROM salesmen, customers WHERE salesmen.salesman_id = customers.salesman_id;
SELECT *  FROM salesmen JOIN customers ON salesmen.salesman_id = customers.salesman_id;
SELECT *  FROM salesmen LEFT JOIN customers ON salesmen.salesman_id = customers.salesman_id;
SELECT *  FROM salesmen RIGHT JOIN customers ON salesmen.salesman_id = customers.salesman_id;
SELECT *  FROM salesmen FULL JOIN customers ON salesmen.salesman_id = customers.salesman_id;

JOIN tables with relationships seems not making any sense

So, if tables have one-to-many relationship, you might as well using the simple one, such as

SELECT *  FROM salesmen, customers WHERE salesmen.salesman_id = customers.salesman_id;

I might say using JOIN with tables with no relationship is more productive?