Note: please see here for an animated visual of how inner joins work.

-- IMPLICIT INNER JOIN

SELECT * FROM customers, orders 
WHERE customers.id = orders.customer_id;

-- IMPLICIT INNER JOIN

SELECT first_name, last_name, order_date, amount
FROM customers, orders 
    WHERE customers.id = orders.customer_id;

    
-- EXPLICIT INNER JOINS

SELECT * FROM customers
JOIN orders
    ON customers.id = orders.customer_id;
    
SELECT first_name, last_name, order_date, amount 
FROM customers
JOIN orders
    ON customers.id = orders.customer_id;
    
SELECT *
FROM orders
JOIN customers
    ON customers.id = orders.customer_id;

-- ARBITRARY JOIN - meaningless, but still possible 

SELECT * FROM customers
JOIN orders ON customers.id = orders.id;