SELECT

    FROM clients
    WHERE client_id NOT IN (
    SELECT DISTINCT client_id
    FROM invoices
    )
    从客户表内选择定了生菜的客户
    image.png
    子查询方法：
    SELECT
    customer_id,
    first_name,
    last_name
    FROM
    customers
    WHERE customer_id IN (
    SELECT o.customer_id
    FROM order_items oi
    JOIN orders o USING (order_id)
    WHERE product_id = 3
    )
    *连接    方法：
    SELECT DISTINCT customer_id, first_name, last_name
    FROM customers c
    JOIN orders o USING (customer_id)
    JOIN order_items oi USING (order_id)
    WHERE oi.product_id = 3