How to group by 2 fields and order by date at the same time?

问题

I have 3 fields in an employees table: personal_id, personal_id_type, and date_registered.

I need to find all the rows, one row for each personal_id + personal_id_type (here is where I think I can use a GROUP), but since there may be many entries for the same personal_id + personal_id_type (because employees can quit and get hired again later), I need to fetch only the newest entry (order by date_registered DESC).

Is this conceptually correct? What would be the syntax for grouping by 2 fields? Will I achieve what I need with that?

Edit: I'm sorry, actually there are a lot of fields, so I need the newest row, not only the newest date.

回答1:

Find the newest date for every (personal_id, personal_id_type) combination in a subquery, then join to the original table:

SELECT
    e.*
FROM
    employees e
  JOIN   
    ( SELECT
          personal_id
        , personal_id_type
        , MAX(date_registered) AS latest_date_registered
      FROM
          employees
      GROUP BY
          personal_id
        , personal_id_type
    ) AS grp
    ON  grp.personal_id = e.personal_id
    AND grp.personal_id_type = e.personal_id_type
    AND grp.latest_date_registered = e.date_registered

回答2:

It sounds like what you want is:

select personal_id, personal_id_type, max(date_registered)
from employees 
group by personal_id, personal_id_type

Conceptually, you are not ordering by date, since you are returning only one per personal_id/personal_id_type combination. Since you are using GROUP BY, you use an aggregate function to select the MAX date.

回答3:

SELECT
    personal_id,
    personal_id_type,
    MAX(date_registered) AS latest_date_registered
FROM
    Employees E
GROUP BY
    personal_id,
    personal_id_type

If you need additional columns then you'll need to add more detail to your question.

回答4:

SELECT personal_id, personal_id_type, MAX(date_registered)
FROM employees
GROUP BY personal_id, personal_id_type
ORDER BY MAX(date_registered) DESC

回答5:

This is where aggregate functions become useful. You will group by personal_id and personal_id_type, then use MAX(date_registered) to select the newest date. Just remember that you have to group by all non-aggregate columns.

回答6:

Yes, that is correct:

SELECT 
    personal_id, 
    personal_id_type, 
    MAX(date_registered) AS latest_date
FROM 
    employees 
GROUP BY 
    personal_id,
    personal_id_type
ORDER BY
    MAX(date_registered) DESC

来源：https://stackoverflow.com/questions/8082379/how-to-group-by-2-fields-and-order-by-date-at-the-same-time