需要使用union返回以下列（3）：
所有没有发票的客户
所有未售出的产品
类别：这与“客户”或“产品”有关吗？打印“客户”或“产品”
id:customer.id（category=“customer”）或product.id（category=“product”）
名称：customer.customer\u name（category=“customer”）或product.product\u name（category=“product”）
table：
顾客
身份证件
客户名称
城市id
客户地址
联系人
电子邮件
电话
产品
身份证件
库存单位
产品名称
产品描述
当前价格
库存数量
发票
身份证件
发票号码
客户id
用户\帐户\ id
总价
发布时间
到期时间
支付的时间
取消的时间
退款时间
发票项目
身份证件
发票id
产品id
量
价格
行总价格
到目前为止，有以下几点：

SELECT 
  category,
  CASE
      WHEN category = 'customer' THEN c.id
      WHEN category = 'product' THEN p.id
  END AS 'id',
  CASE
    WHEN category = 'customer' THEN c.customer_name
    WHEN category = 'product' THEN p.product_name
  END AS 'name'
FROM 
  (
    SELECT
        CASE
          WHEN c.id = c.id THEN 'customer'
          WHEN p.id = p.id THEN 'product'
        END as 'category'
    FROM
        customer as c
    LEFT Join -- Left join to show all customers even those with & without invoices
        invoice as i
    ON c.id = i.customer_id
    AND i.id IS NULL -- Gives me all customers who do not have an invoice
    JOIN invoice_item as ii
    ON i.id = ii.invoice_id
    Join product p
    ON p.id = ii.product_id
  ) tb1

UNION ALL

SELECT 
  category,
  CASE
      WHEN category = 'customer' THEN c.id
      WHEN category = 'product' THEN p.id
  END AS 'id',
  CASE
    WHEN category = 'customer' THEN c.customer_name
    WHEN category = 'product' THEN p.product_name
  END AS 'name'
FROM
  (
    SELECT 
      CASE
        WHEN c.id = c.id THEN 'customer'
        WHEN p.id = p.id THEN 'product'
      END as 'category'
    FROM
        product as p
    LEFT JOIN -- Left join to show all products even those that sold and not sold
        invoice_item as ii
    ON p.id = ii.product_id
    AND ii.invoice_id IS NULL -- Gives me products that didnt sell
    JOIN invoice as i
    ON ii.invoice_id = i.id
  ) tb2

开放的任何建议，因为我一直在试图找出如何显示类别作为“产品”或“客户”。提前谢谢！