选择有发票的客户
    SELECT
    FROM clients
    WHERE client_id IN (
    SELECT *DISTINCT     client_id
    FROM invoices
    )

    SELECT
    FROM clients c
    WHERE *EXISTS     (
    SELECT client_id
    FROM invoices
    WHERE client_id = c.client_id
    )
    image.png
    image.png

    image.png
    找到那些从来没有被订购过的产品
    SELECT
    product_id
    FROM products p
    WHERE NOT EXISTS(
    SELECT product_id
    FROM orders
    WHERE product_id = p.product_id
    )