根据条件从另一个 table 获取计数

Question

我有三个 table，我想查询所有公司。但是我想通过从 people_history table 中获取每个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活跃员工的数量。

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，我不会查找每个人的最后一条记录。

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

有什么建议吗？

谢谢！

Answer 1

这很容易通过反连接解决：

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

这只计算 person_history table 中每个人的最后一条记录（最高 ID）。

Answer 2

未测试但关闭工作：

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

根据条件从另一个 table 获取计数

Getting the count from another table based on a criteria

mysql

sql