根据条件从另一个表获取计数

Question

我有三个表，我想查询一下所有公司的信息。 但是我想通过从people_history表中获取每人的最后一条记录并将其与他们工作的公司进行匹配来了解在职员工的数量。

Table: people

—————————————————————————————————————————————————
   id   |    name     |    ssn   |    phone     |
—————————————————————————————————————————————————
   1    |    John     |   9591   |   12341234   |
   2    |    Jane     |  1049    |   12340987   |
—————————————————————————————————————————————————

Table: people_history

—————————————————————————————————————————————————
id  |   person_id   |  company_id  |   time     |
—————————————————————————————————————————————————
1   |       1       |       5      |   stamp    |
2   |       1       |       7      |   stamp    |
3   |       2       |       2      |   stamp    |
4   |       1       |       2      |   stamp    |
—————————————————————————————————————————————————

Table: companies

————————————————
id  |   name   |
————————————————
1   |  Name 1  |
2   |  Name 2  |
…              |
————————————————

通过执行以下操作，我不会寻找每人的最新记录。

SELECT c.name AS company_name, COUNT(h.id) AS employees 
FROM companies c 
LEFT JOIN people_history h ON h.id = c.company_id 
GROUP BY c.id

有什么建议么？

谢谢！

Answer 1

未经测试，但关闭了工作：

SELECT c.name AS company_name, COUNT(h.id) AS employees 
FROM companies c 
LEFT JOIN (SELECT * 
           FROM people_history ph1 
           WHERE id = (SELECT MAX(id) 
                       FROM people_history ph2 
                       WHERE ph1.person_id = ph2.person_id)
           ) h ON h.id = c.company_id 
GROUP BY c.id

Answer 2

使用反联接很容易解决：

SELECT c.name AS company_name, COUNT(h1.id) AS employees 
FROM companies c 
LEFT JOIN people_history h1 ON h1.id = c.company_id
LEFT JOIN people_history h2 ON h2.id = c.company_id AND h2.person_id = h1.person_id AND h2.id > h1.id
WHERE h2.id IS NULL
GROUP BY c.id

这仅对person_history表中每个人的最后一条记录（最高ID）进行计数。