Mysql Count相关记录不计算在内

Question

我有一个用于库数据库的多个表，我想要计算已经向某个组提供了多少个出版物。 我有这些表
顾客表

CREATE TABLE `patrons` (
  `id` int(10) UNSIGNED NOT NULL,
  `category_id` int(10) UNSIGNED NOT NULL,
  `firstname` varchar(255) NOT NULL,
  `lastname` varchar(255) NOT NULL,
  `email` varchar(255) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

赞助人类别

CREATE TABLE `patron_categories` (
  `id` int(10) UNSIGNED NOT NULL,
  `name` varchar(255) NOT NULL,
  `overdue` float NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

出版物

CREATE TABLE `publication` (
  `id` int(10) UNSIGNED NOT NULL,
  `name` varchar(255) NOT NULL,
  `year` year(4) NOT NULL,
  `ISBN` varchar(255) NOT NULL,
  `type` varchar(24) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

结帐

CREATE TABLE `checkouts` (
  `id` int(10) UNSIGNED NOT NULL,
  `publication_id` int(10) UNSIGNED NOT NULL,
  `patron_id` int(10) UNSIGNED NOT NULL,
  `checkout` date NOT NULL,
  `checkin` date NOT NULL,
  `actual_checkin` date DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

这是我的查询，列出赞助人类别及其总顾客及其结帐的当前出版物数量。

SELECT pc.NAME                 AS Category, 
       Count(p.id)             AS patrons, 
       Count(c.id)         AS publications 
FROM   patron_categories AS pc 
       JOIN patrons AS p 
         ON pc.id = p.category_id 
       LEFT JOIN checkouts AS c 
              ON c.actual_checkin = NULL 
       LEFT JOIN publication AS pub
              ON c.publication_id = pub.id 
GROUP  BY Category
ORDER BY Category

我不知道出了什么问题，但它给出的出版物数量为0，而有许多出版物借给了那些顾客。 请帮忙。 具有虚拟数据的完整数据库位于here。

Answer 1

除了检查c.actual_checkin字段是否为空之外，您的查询看起来很好。尝试将join语句更改为ON c.actual_checkin = NULL到ON c.actual_checkin IS NULL

Answer 2

试试这个：

SELECT pc.NAME                 AS Category, 
       COUNT(p.id)             AS patrons, 
       COUNT(c.id)         AS publications 

FROM   patron_categories AS pc 
       JOIN patrons AS p 
         ON pc.id = p.category_id 
       LEFT JOIN checkouts AS c 
              ON c.patron_id =  p.id
       LEFT JOIN publication AS pub
              ON c.publication_id = pub.id 
GROUP  BY Category
ORDER BY Category

结果：