Getting the count from another table based on a criteria

Question

I have three tables and I would like to make a query of getting all companies. BUT I would like to know the amount of active employees by getting the last record per person from the people_history table and matching it with the company they work for.

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  |
…              |
————————————————

By doing the following I don't look for the last record per person.

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

Any suggestions?

Thank you!

Answer 1

Not tested but shut work:

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

This is easily solved with an anti-join:

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

This counts only the last record (highest id) for each person in the person_history table.