PostgreSQL: how to get max() of GROUP BY?

Question

I have a table called history with the fields id , lastname , event and date .

It saves events of persons that works in my office like "Entering office", "Exiting office", "Illness", "Holidays", etc.

I need to get the last event of every person.

This is what I tried but it doesn't work:

SELECT lastname, event, max(date)
FROM personshistory
GROUP BY lastname, event;

Answer 1

You can use distinct on :

select distinct on (lastname) ph.*
from personshistory
order by lastname, date desc;

distinct on is a very convenient Postgres extension. It keeps one row for each value of the expressions in parentheses. The specific row is determined by the order by clause -- based on the keys that follow the distinct on keys.

Answer 2

With NOT EXISTS:

SELECT p.lastname, p.event, p.date
FROM personshistory p
WHERE NOT EXISTS (
  SELECT 1 FROM personshistory
  WHERE lastname = p.lastname and date > p.date 
)

or join your query (without the event column) to the table:

SELECT p.lastname, p.event, p.date
FROM personshistory p INNER JOIN (
  SELECT lastname, MAX(date) maxdate
  FROM personshistory
  GROUP BY lastname
) g on g.lastname = p.lastname and g.maxdate = p.date