Mysql 如何在不真正连接该表的情况下，通过连接获取某个字段值？

嗯，我有一个来自客户机的数据库，其中的表没有良好的关系，是一团乱。获取所需信息的问题是，我需要关联表的字段与我将要使用的表不相关

我要说清楚：

共有3张表格：参观、咖啡和付款

桌上咖啡有付款id。 餐桌上有咖啡。 表付款没有ID

我想知道有多少客人喝了咖啡，每杯咖啡付了多少钱

结果应该是：

访问|咖啡|付款
8234 |摩卡| 829

那么，我怎么知道所有的付款，咖啡是什么，有多少次访问

可能是显而易见的，但我也被同样的事情弄得不知所措

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谢谢大家的帮助

要获得每种咖啡的访问：

SELECT c.name AS coffee, IFNULL(count(v.coffee_id), 0) AS visits
FROM coffees AS c
LEFT JOIN visits AS v ON c.id = v.coffee_id
GROUP BY c.id

SELECT c.name AS coffee, IFNULL(count(p.id), 0) AS payments
FROM coffees AS c
LEFT JOIN payments AS p ON c.payment_id = p.id
GROUP BY c.id

要获得每种咖啡的付款：

SELECT c.name AS coffee, IFNULL(count(v.coffee_id), 0) AS visits
FROM coffees AS c
LEFT JOIN visits AS v ON c.id = v.coffee_id
GROUP BY c.id

SELECT c.name AS coffee, IFNULL(count(p.id), 0) AS payments
FROM coffees AS c
LEFT JOIN payments AS p ON c.payment_id = p.id
GROUP BY c.id

要在一个查询中同时获取它们，请执行以下操作：

SELECT coffee, MAX(visits) AS visits, MAX(payments) AS payments
FROM (
    SELECT c.name AS coffee, IFNULL(count(v.coffee_id), 0) AS visits, 0 AS payments
    FROM coffees AS c
    LEFT JOIN visits AS v ON c.id = v.coffee_id
    GROUP BY c.id
    UNION
    SELECT c.name AS coffee, 0 as visits, IFNULL(count(p.id), 0) AS payments
    FROM coffees AS c
    LEFT JOIN payments AS p ON c.payment_id = p.id
    GROUP BY c.id) x
GROUP BY coffee

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如果payments表没有id，payments\u id用于什么？我的意思是，payments只有id字段，但没有像coffe\u id这样的关系。例如，向我们展示3个表的剪切粘贴错误模式加上示例数据。听起来模式

可能足够了，但如果没有任何与表相关的内容，您就不知道db工作，没有办法计算你想要什么。