MySQL 列出单个表中的一些字段并计算来自另一个表的所有特定字段引用

Question

我的问题的主题有点令人困惑，因为我已经没有足够的词来适应它的一般和技术描述了。

用不太专业的术语来说，我要具体完成的是:

List all churches and their corresponding booking status from a service they are offering with the following fields:

• 教会 ID AS id
• 教会名称AS name
• 他们提供的所有服务的pending 状态计数 AS pendingCount
• 他们提供的所有服务的完全预订状态计数 AS bookedCount

关联表的结构如下:

教会

+---------+---------------+
| id(int) | name(varchar) |
+---------+---------------+

服务

+---------+---------------+---------------+
| id(int) | name(varchar) | churchId(int) |
+---------+---------------+---------------+

预订

+---------+-----------------+----------------+
| id(int) | status(varchar) | serviceId(int) |
+---------+-----------------+----------------+

到目前为止我得到的是这个，我完全不知道为什么它正在编译但会产生负面结果:

SELECT
    churches.id,
    churches.name,
    pending.count AS pendingCount,
    booked.count AS bookedCount
FROM
    churches
INNER JOIN
    services
ON
    services.churchId = churches.id
LEFT JOIN
    (SELECT
        COUNT(bookings.id) AS `count`,
        bookings.serviceId
     FROM
        bookings
     WHERE
        bookings.status = 'pending'
     GROUP BY
        bookings.serviceId)
     AS pending
ON
    pending.serviceId = services.id
LEFT JOIN
    (SELECT
        COUNT(bookings.id) AS `count`,
        bookings.serviceId
     FROM
        bookings
     WHERE
        bookings.status = 'fully booked'
     GROUP BY
        bookings.serviceId)
     AS booked
ON
    booked.serviceId = services.id

示例输出

Answer 1

我会为此使用条件聚合。这是一种对满足特定值的行数求和的方法。在这种情况下，我们可以对 SUM(b.status = 'fullyBooked') 和 SUM(b.status = 'pending') 使用聚合并按教会分组 id 像这样:

SELECT c.id, c.name,
    SUM(b.status = 'pending') AS pendingCount,
    SUM(b.status = 'fully booked') AS bookedCount
FROM bookings b
RIGHT JOIN services s ON s.id = b.serviceId
RIGHT JOIN churches c ON c.id = s.churchid
GROUP BY c.id;

重申一下，通过使用 SUM(condition) 并按某个 id 值分组，您计算的是该 条件为真的次数id 值。

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编辑:

我已将 RIGHT OUTER JOIN 添加到此查询中，这样也将返回没有任何预订的教堂。